Your size demands really hamstring you into very expensive and modern cuts. Vintages were never really cut 3" wide unless they were also ridiculously long (14x3" and the like). a 8x3" is the closest to your size requirements and that's in the $300 range from ardennes last I checked... likely much, much, MUUUUUCH more if you're wanting a la veinette or some other valued vein.
La Grise are far from the worst vein to learn on, but they tend to be more trouble than most. If you really want to go coticule without spending $500 on a stone that you probably will begrudge for its expense, I'd suggest getting comfortable on 1.5" and 2" wide stones. They're the standard for basically every razor hone out there. Jnats allow some exceptions, but even they seem to be cut under 3" wide for razors as often as not.
Short of that, your best bet is a 5x2.5" vintage. The glued ones don't go crazy like the natural combo's tend to on eBay ($125-175 instead of $200-350), so they can be gotten reasonably... and the real estate is as close as you're going to get to what you're looking for without breaking the bank.
Coticule Suggestions