I have a 3 tier cake for this Sunday and I don't want to spend my whole weekend decorating/baking. so my question is, can I bake today, seram wrap them today, tomorrow (Thursday) add simple syrup and frost and then refridge and decorate on Friday and let the cake sit in the fridge till sunday?

You can 100% bake ahead.  I always do.  I also make my buttercream and fondant ahead of time as well.  The freezer is your friend.  Plus a cake that has been frozen tends to be more moist then one that hasn't.  So go ahead and bake today, level your cakes and wrap them well and stick them in the freezer.  You could even bake, level, fill and crumb coat then wrap and place in the freezer until you are ready to decorate on Friday if you have enough room in your freezer

can I skip the freezing part just because I'm adding simple syrup and want it completely done, by Friday night.

Jenmariene, you can skip the freezer, but the point is to preserve the freshness of the cake.  Keeping the cake refrigerated will help, but is no substitute for the freezer.