in ai >> Extrude text that has background-pattern with transparency

I use ai rarely... more photoshop... excuse me up front for any dumb mistakes :p

I want to create characters (ABC..) with nice textures, a stroke and a 3d extrude effect.

The problem is that some of my chosen textures have transparent parts, so when on a character I execute: effect -> 3d -> extrude & bevel you can see through the transparent parts of the front-face to the extruded borders of the character.

I don't know how to fix that. In photoshop I would first create white characters behind the pattern-textured characters, then merge the two so that it's one character with a pattern that has it transparent parts replaced with white color and then extrude. But it's ai and I don't know how to do this kind of layer merge with objects. But maybe this is also a wrong approach ?

Is there a solution ?