Yeah, nobody sells a 3000 microgram hit of acid. That's easily enough material to make 40 average-sized hits.
There are substances that are commonly dosed around 3000 mcg, like DOC or 5-MeO-AMT; it could also be an NBOMe or NBOH, although a gelcap would be a less than optimal way of administering one of these, because oral administration (i.e. immediately swallowing it) tends to drastically decrease their bioavailability... maybe that's why it's dosed at 3000 micrograms, when NBOMe's are usually dosed below 1000 mcg. However, selling it like that would also be extremely irresponsible, because when administered sublingually/buccally/intranasally, 3000 mcg of an NBOMe could easily result in a life-threatening overdose.
Or who knows, maybe your dealer just pulled the "3000 mcg" figure out of his ass. At any rate, "LSD in a gel cap" is a pretty huge red flag.