Blizzard added that the plans for the multiplayer component are unchanged by the splitting of the campaigns. However, some units will now be unique to the campaigns and will not be playable in multiplayer.
I would say this means all three games have identical multiplayer, including units. The unique units in single player are nothing new (starcraft had them too, like raynor, duke). It seems like the only content being "split" is the campaign, so maybe the second and third titles will be quite cheap, especially if downloaded. Possibly they'll have discounts for people who bought the terran box too, so who knows?