Is this a hard cap or will we be able to eventually unlock more character slots through the marketplace or other methods?
Most likely through marketplace. But why would you need more than 3? You can unlock everything on one character.
I'm pretty much playing just one toon (Hamster) and the reason I'd like to have other characters (even if they just get a trickle of cert points over time) is to occasionally be able to unlock something that I'm not sure if I want to invest on with my main and give it a try.
THERE IS NO NEED TO HAVE MORE THAN 3. You play a different faction on a different server. And each character can do EVERYTHING. Period. I can understand wanting a character on every server that exists, but right now why spread yourself so thin? Its already going to take a few months of daily play to get ONE character set up pretty decent, why waste time?
Wait, what? First, you can always use the Trial to test an item for a day. But beyond that, instead of spending certs on your main character you're going to go out with a secondary character, earn the certs on him, then spend them. I mean, seriously, you could have spent that time playing with your main character and earned the same number (or more) certs for him, tested the item on him. What's the difference? You're still wasting your time grinding away to earn certs, right? So does it really matter if you earn then on character A or B? And if you're faced with the choice of which character to grind away on wouldn't you rather choose A (the main character) as any versatility you can give him will help in the long run, even if you don't use the versatility very often?