I have or have had all three in quite a variety of calibers. All are good but as others have said the Verney-Carron is at least a cut above if not a few cuts above either the Merkel or the Chapuis.
As mentioned by others, if at all possible try to see and handle all three side by side. The first thing you will notice is the V-C looks like the hand-built double it is, the other two look more like assembly-line productions.

All three are safe, well-built, accurate and functional double rifles. Verney-Carron doubles are all that and more.

Good luck and stay well