Right now I have 6 characters in my party, should I be concentrating on biulding up just three since I can only use three at a time and just leave the others to the side, or should I be rotating them all equally so that they all increase in experience at the same rate?

Different individuals have different opinions on that. Some have been able to take down the biggest threat in the game with only 3 high-level characters (check Youtube). Others, like myself, recommend leveling them equally as it will help you be more flexible in battle, allowing your back up characters to survive any blows they might take while you wait for them to trigger a quickening or revive any K.O.'d characters.

No matter what you choose to do, make sure to pick up some Embroidered Tippets when you reach Old Archades, as they give you double EXP, which can help catch up lagging members, or hasten the leveling of your chosen party of 3.

Always try to balance your characters, if the primary 3 fall (The ones you start a fight off with) and the creeps are high levels, you're practically dead when you send the weaker ones in.

The only character I have in my party %100 of the time is Vaan, and that's only because I've gotten used to having one main character. I am constantly swapping my other characters around and making sure they're reasonably equally equipped.

Then again, Knegative is totally right, it is a preference thing. If you'd rather have 1 super-strong team, thats entirely up to you.

But alexrider has a good point as well, if you have a super-strong team of 3, what happens if they fail? Unless they're almost invincible, you're practically digging yourself into a grave.

According to the guide have all characters balanced to lvl 40-45 and then pick 3 characters and focus mainly on them. I didn't follow this though and it worked out fine. I borrowed the guide didn't buy:D....

Pick how you like to play choose either to Level only 3 like I did or all of them it really doesn't matter if you have good Equips and Gambits.

The basic Gambit Ally any Phoenix down on all characters really makes having a balanced party moot since Items like that espically if it is on the top of the list happen almost as soon as the character hits the ground.

I have only had problems in 2 areas Level death spells and Zodiark's special. Zodiark because his special can randomly kill one two all or none of the onscreen characters. While Level death only work if the characters are all the same level just have them each different levels and no problem.

So really just play as you want.

i like to keep vaan 4 levels above the rest. right now he is at 81 the rest are at 77.. do teams of 2 or 1 to make it go a bit faster. especially in giruvegan and places like that. deathbringer(1 hit kill), excalibur help alot. even more so than zodiac spear in some cases.