don't Know who to buy

I don't know if you could have picked 3 more champions with greater variance in their playstyles. Irelia builds into a tanky fighter who can initiate relatively well; Brand is a bursty AOE mage who bats cleanup but has to wait on an initiation; and Jayce is a poke-siege master who thrives when there IS no initiation.

I say all that to say this: it depends heavily on your own playstyle. Pick the champion with whom you feel like you gel better.

Like getting in the enemy's face and zoning squishies out of the fight? Irelia.

Like melting faces, especially multiple faces at once? Brand.

Like making your enemies feel like they can't even hide under their tower? Jayce.

Of course, these are all generalized and, in the proper hands, these champions can do more than what I've pointed out, but these are their greatest strengths. My advice would be to determine how you like to play then pick a champion according to that preference.