Hopefully, when they recharged, they put in some UV tracer dye. A savvy mechanic would have done that.
$3000 seems high. For that much money, they would (or should) change nearly everything.
It appears that you do have a leak. This leak needs to be found before a reasonable quote can be offered. He should be allowed to do some preliminary work before the actual repair. Are you willing to do some work yourself? Throw some soapy water on all the connections and look for bubbling. Then you can give that information to the mechanic for a reasonable quote.
95 E320 Cabriolet, 131K