Good advice so far, hope I don't ruin the streak.
Cheapest if it can be found, is a police turnin 870 or 500. Probably ravaged on the outside, near new inside. Pick it up for less than $200 and look for a longer barrel when you can afford one.
The shorter barrel works fine for casual clays and skeet, though trap requires more choke than IC or none, the two most common police choices.
Combos are rare on the used shotgun mart. If one comes along with a short barrel,either bead sighted or with open sights, and a long barrel with vent rib and choke tubes, grab it forthwith.
Don't worry about 3" capability now. 2 3/4" shells will suffice for everything mentioned.
HTH...