I am going to offer some suggestions for going forward, rather than things you can do right now. You can’t do much of anything with 500 gp, and no feat slots available until 6th level. You might consider discussing the 500 gp with the DM: the total value of your items should be between 2,700 gp and 5,400 gp at your level. While the Dungeon Master’s Guide presents wealth by level as a guideline, and it can be deviated from, it is close to a bare minimum for many classes. Going significantly under WBL is a very dangerous thing to do and should be done very carefully. Of course, if you mean you have 500 gold pieces, but you also have 2,200-4,900 gp worth of assorted other gear, things are just fine. Money is tight at these levels, after all.
Armored AC – some pointers, but most of all don’t worry about it too much
First, keep in mind that AC, particularly armored AC, is of limited value. You have the largest HD in the game, and excellent Constitution. Armored AC protects you almost-exclusively from HP-damage, which you should be equipped to handle.
Second, medium armor tends to be really bad, and burning a feat on Heavy Armor Proficiency is worse. Unless you plan on multiclassing to something that gets that for free, you should probably stick with a masterwork chain shirt. That is, at least until you can afford mithral full-plate, which you cannot with 500 gp. The other option, masterwork breastplate, triples the armor check penalty for just +1 AC; I do not recommend this.
On the other hand, once you can, mithral full-plate offers +8 AC, double the chain shirt, and while it has the same −3 armor check penalty, +4 AC is far more worth that than is +1 AC. Depending on your game, you might instead get mithral hellknight plate; this is, in Pathfinder’s default setting Golarion, something of a uniform for the hellknights, so it would piss them off quite a lot if you (a non-hellknight) were to wear it. But if you are not playing in Golarion, the armor either may not exist, or may exist without the strings attached. Talk to your DM about it when you can seriously consider getting 10,500-11,000 gp armor (not until 7th-8th level, if your DM keeps to the WBL guidelines).
A shield can help; a masterwork heavy shield is +2 to armored AC for a little gold and a minor armor check penalty. A two-hander is usually nearly-mandatory for a melee warrior, but you have maneuvers that can go a long way to resolving that, and even with your great Strength for your level, the extra ½ Strength only represents +2 damage.
With both armor and shield, the mandatory +1 enhancement bonus on each means you’ll get another +2 from that. Talk to your DM before upgrading your masterwork chain shirt to a +1 chain shirt – since you eventually want to swap it out for mithral full-plate, you need to know how he’s going to handle your wealth. By the rules, he’s supposed to make sure your wealth gets back up to the written levels after your sale (by increasing loot in the future to make up for the 50% loss on the sale), but your DM already seems to have drastically reduced the wealth of your game, based on your 500 gp at 3rd level.
Touch AC – more important, much more expensive
Unlike armored AC, touch AC is fairly valuable. It’s also much more difficult to increase, and 500 gold won’t do much for you. Usually, gloves of dexterity are a good idea for a warblade eventually, but are lower priority than a belt of giant strength or amulet of vitality. With your massive Strength and Constitution, but weak Dexterity, I might spring for a bit of that Dexterity early. Eventually, gloves of dexterity +6 will nicely fill out the +3 maxmimum Dexterity rating on mithral full-plate, but that is a long way in the future.
Note that wall of blades could be an option for you here. It’s limited, but once every few rounds you can have a strong chance to negate one attack.
Saving Throws – Diamond Mind is your friend
You really do want to keep moment of perfect mind available... most of the time. Will saves are a weakness for you, and moment of perfect mind is “you will almost-certainly make this Will save” once every few rounds.
Action before thought and mind over matter are far less important, since Reflex is just a less-important saving throw and your Fortitude is already beastly.
Aside from these things, just keep your cloak of resistance up to date; it doesn’t always have to literally be the most expensive version of it you can afford... but you should buy the most expensive version you can afford pretty soon after you can afford it, like in a level or two.