Class: Expert Droid
T3-M4 is only required for one specific task throughout the entire game, and apart from breaking open that single door, he probably won't see much action for the rest of your playing time. No one can really match him when it comes to the gear head skills of computer use, repair, and security, but he excels in skills at the cost of power in battle; he gets around eight vitality per level, which isn't horrible, but he can't use any weapons except for blaster pistols during combat, which obviously will hamper his damage output. He does possess the ability to use the special droid equipment, like flamethrowers and stun rays, but this doesn't make him any more special than HK-47, who in general is a more proficient warrior than T3-M4.
Attributes: Since his intelligence is already stratospheric, and you can boost the wisdom-based skills with equipment, you should focus on dexterity for T3; it'll combine well with his combat logic upgrade to increase his defense, and help his chances of connecting when using his blaster pistol.
Skills: Obviously, add to computer use, repair, and security. It's not uncommon to see some of these skills get above 30 by the end of the game, with the proper equipment.
Feats: Don't worry about blaster pistol focus; you'll find targeting computer equipment that will give T3 this feat, and perhaps even specialization, soon enough. Instead, go for gear head to max out his skill potential, followed by either toughness or two-weapon fighting, depending on your tastes.