Defensive strategy

Im quite new myself, what i would say is that hangar bays and repair bays are the best for a static defense. Choke planets are the ones you want to really beef up with static defenses, you know ones that are highly contested with lots of phase lanes. Also phase stabilisers are very handy 2 or 3 of those work wonders. Also having a small defensive fleet works well too the benefit is obviously freeing up your main fleet to go out on the offensive. What ive found in my short time playing is that about 4 hangar bays 2 - 3 repair bays and about 3 or 4 turrets makes the computer think twice about attacking, on hard you might need a defensive fleet there for him to turn around as soon as they turn up. Hope that helps

Also upgrading your emergency facilities increases the health of your planet if for some reason you dont have defenses there yet.

With regards to spreading your fleet too thin, in my limited experience i find that having a defensive and offensive fleet works well but it only really works against one AI because if your fighting 2 AI's and you only concentrate on 1 and only 1 then the other AI is going to get alot stronger and eventually attack you with a large fleet as i found out recently!! I hope that helps, enjoy playing

Defensive structures were never designed to hold their own against a dedicated invasion fleet (with the exception of the upgrades in Entrenchment - and pirates usually don't do enough damage if you have a good mix of structures). They do however serve well as a way to buy your fleet time to get there and (hopefully) save your planet. Phase Jump Inhibitors also help slowing their escape and allows you to pick them off (which is even better if you have the Space Egg with Gravity Warhead - the extra charge time required is far longer than the 25 second cooldown).

well, what I do like to do is, as a first step, get a repair station and a few turrets in its vicinity and upgrade the planet's emergency facilities. if you place your logistics structure in that cluster also, then at least you have a little time to react and get a proper defense in place.

that is what I do for border worlds and possible one jump further early on. depending on the map setup and how much I get attacked, I get tactical upgrades and sation more repair centres, canons and hangars and might upgrade the planet even more.

fleet-wise, I usually split them up a bit, rarely ever do a complete steam roller. it depends on the size and layout of the map, but I try to station my fleets such that I can reach every one of my worlds in one, or at the maximum 2 jumps with one of my fleet groups. that usually leaves me with 2 - 3 fleets in medium sized games. when in the offensive, I then usually take most of that 'sector's' fleet to attack and leave defensive tasks to the resupply that has since been produced and stationed.

that said, one of the best defenses is still recon. knowing, or at least having a good idea, when and where the enemy is likely to strike is invaluable.

I find turrets more effective than hanger bays as static defense and have of late cut back to no more than 2 hanger bays.  The objective is to slow em down until you can get a fleet in there and the turrets seem to do a better job in this regard because you can throw a bunch up.  Augment those turrets with a couple repair bays in appropriate spots and you gain quite a bit of time to respond.  Building up emergency facilaties along with techs that reduce bombardment damage give you extra time as well. 

The concept of using your main fleet to focus on taking down one opponents offensive capability while a smaller fleet works on the other opponents economy seems a good one.  A small fleet consisting of high DPS units like missile frigs augmented with a couple repair ships is quite effective and can chew up resource gathering structures and trade structures pretty quickly.

I tend to favor building stronger static defenses which allows me to have a single strong response fleet covering planets 3 and 4 links away instead of 1 or 2 links away

Though I have not played a game against 2 hard opponents with locked teams, I would think the key is to stike one opponent agressively as early as possbile even at the expense of your own expansion.  It would seem difficult to win against 2 hard team locked opponents by just focusing on rapid expansion.

As you pointed out, things went well for awhile with everyone building up...but now both of your enemies have built to the point where they have a significant offensive punch.

Looks like I have something to try tonight myself

If you can afford it, continue to build defenses during the attack. I tend to start building new hanger bays on the other side of the planet as the old ones get razed. It's amazing how long a few strike craft will hold off a fleet, unless it has a strong Kol or something in it. I also tend to, if at all possible, not bother saving my planets. I mean, I'll gladly let my planet get slowly ground down if it means that the ships attacking it are "neutralized" temporarily, because then I can attack myself. Usually, by the time the attacking fleet starts turning around to fend off my attack 2-3 jumps away, I've established a foot hold. They'll arrive after I've cleared the entire gravity well. Unless they decided to try a pincer, I basically "killed" their fleet several minutes prior by holding them with my static defenses and maneuvering to their rear.

I do what I like to call "the Thermopylae approach"

Turrets are more effective than hangars, and I usually max out my logistics slots with about 20 turrets, two or three repair bays, and two or three hangars with fighters and a phase jump inibitor. I mass them all together as tight as possible. The fighters take out any planet killers while any enemy who is within range of even one turret is most likely within range of 10 of more others.

Since the AI only attacks one or two buildings at a time, you dont need very many repair bays. Plus, you can easily rebuild any lost turrets because they build so fast. If the enemy is too large to take heavy losses from your defenses, then they will at least tie them up enough for you to get your main fleet in and crush them as they attempt to flee (hence the PJI)