So i made a new charactor, he's a breten bloodmage (Top skills: Shortblade, Destruction, Alteration, Sneak, Mysticism...)

I wanted to know if it would make sense for him to be in house telvanni, The theives guild, and the morag tong...
Thanks.

What do you think?

Membership with House Telvanni and the Thieves Guild doesn't seem conflicting at all, but I think the Morag Tong frowns upon crime and dishonourable conduct rather than making them a way of life like the other two factions. Unless your character doesn't care about the Morag Tong's rules, kill sparingly and don't expect your writs to absolve you of any recent thefts. Really, a Morag Tong assassin getting caught stealing should cause a scandal, but nothing like that will happen to player character in the game.

yup exactly if you think you can you can do all but if you think you cant well "bye bye" to some of that guilds its you Rp its your character let him say what you want

The Morag Tong is involved in house wars, which means it's likely you'll end up assassinating one or more Telvanni members. You may be able to avoid those quests, I'm not sure.