I"m making 3 8" rounds. 

cutting off the bottom to make a flat surface. 

Putting all three cakes together with buttercream and all  upright with flat surface on cake board. Like a big hill . Just built horizontal istead of vertical.

Will the buttercream be enough to hold them together  or should I run a bubble tea straw through the center of all three. It would be going horizontal. 

thank you for help

If I understand, they are all going together on a cake board and you're worried they'll separate? When I put my cakes on the drum, I glue them down, so no, I wouldn't worry about sliding.
 

AIf it were me I would go the "better safe than sorry" route and secure that cake well if there is any doubt. Yes, the buttercream will probably be enough to hold it, but I like to be really sure. Especially if you need to travel with the cake at all. (Do I understand correctly that you are placing the cake layers perpendicular to the cake board?)

Let me find a pic

AI think I get what you mean. I would secure it, just to be on the safe side. :)

OK. 

Ohhhh...I see. Yes, if you're just using buttercream,  secure the best way you can

OK, thanks for help   everyone :)