Hi CakeChic, I posted this reply to an earlier poster so it might help you also: http://cakecentral.com/t/84/roses-stuffed-between-tiers
From the pic you posted, these look like fresh flowers that could wilt out of water, so I would use an Oasis (florists' foam) separator and dish between the tiers (spray it with water thoroughly before arranging your flowers, no need to dunk it in a sink full of water, it gets oversaturated). Use dowels around the separator dish to hold it in place in the centre of the cake tier.(Make sure the width of the separator is 2 inches less that the cake tier above).
Transport your cake tiers separately with the flowers arranged in the separators and secured in place on the cake. Then stack the cakes at the venue. Hope this helps!