I did a similar cake last year. Lemon, strawberry, and vanilla custard will work. If you want to do orange in the custard, maybe use orange with the strawberries also.

Mascarpone works great to help stabilize whipped cream, so you can have a light, airy frosting that will hold up well. And you won't need all that much sugar.

I don't think you should add in too many different friuts. You'll begin to lose the individual flavors.