I've Googled your triple berry cake filling and cannot seem to find it, are you willing to share the recipe?
I'm sure there is a recipe somewhere out there, but this is a concoction I made up. To make the triple berry cream filling, I melted some strawberry jam (and strained the solids out of it), then melted some seedless raspberry jam and combined them in a small bowl. I then added a little Chambord (blackberry liqueur) until I liked the flavor (not too much alcohol), and let the mixture cool somewhat. I then placed about 1 cup (measurements are NOT exact) of the vanilla buttercream into a small bowl and added the berry mix until I was satisfied with the flavor and texture. I piped some of it around the top of the second layer and filled it with the rest, smoothing it with a small offset spatula until it was smooth. I put the top layer on the cake and crumb frosted it, put it in the fridge for a while to set, then frosted as usual. Hope this helps you! Thanks for asking!
P.S. If you make too much of the berry mixture, you can put it into a jar in the fridge. It will keep indefinitely.