T3 Heavy Assault Bots and T3 Siege Assault Bots fulfill the same role - that of a main-line assault bot. Put simply, the continuation of direct fire units from T1 and T2 (tanks etc). They typically have high health, may possess a personal shield, and have good DPS. Their speed tends to be average. They all have reasonable rate of fire, meaning that their damage scales well, from T1 right up to T3.
The UEF and Cybran Bots, while named differently, are actually very similar in role - they cost the same energy and mass, and have the same build time. They have the same speed--among land units, they are second only to T1 land scouts --but the Cybran Bot - the Loyalist has a longer range and more DPS than the UEF bot - The Titan. However, the Titan possesses a personal shield - giving it more overall health (although its health is split between shield and actual health, meaning it has less of either). This maintains the trend of the Cybran unit having more DPS, but being more fragile. In addition to this, upon death the Loyalist fires its EMP weapon, which paralyzes nearby enemy units for a brief moment. Loyalists are also armed with a tactical missile deflector - making it almost impervious to Mobile Missile Launcher fire.
The Aeon Heavy Assault Bot - the Harbinger Mark IV is a step above the other two in terms of performance, costing almost twice the mass, with half again the build time. The Harbinger is equipped with an engineering suite, allowing it to reclaim units on the battlefield, with a production speed 3/5 that of a T1 Engineer. It does not have the ability to initiate construction, but may be used to aid it, as well as repair other units, although it does each of these tasks slowly. It is also equipped with a powerful personal shield, nearly twice as effective as the Titan's. The Harbinger also has a longer range than either the UEF or the Cybran bot, and inflicts over twice the DPS - making it highly effective at taking on just about any unit.
The Cybran and the UEF both have T3 Armored Assault Bots - The Percival and the Brick, which mean that, unlike the Aeon, these factions often field other direct fire units when at T3. The Aeon's Harbinger isn't quite on the same level as these because of its health, but its DPS is comparable. The Seraphim do not have any form of T3 "main combat" Bot (they do have a T3 Sniper Bot), but instead have a T3 Siege Tank - a unit comparable to the Aeon Harbinger Mark IV.