A nice gun for a starter gun would be a Winchester Select Energy in 32" barrels. 30" barrels would also work. If you could find one with adjustable comb, it would be a bonus for shooting trap, clays and skeet.
If you are patient, you should find one for around $1,000 or less.
For a little under $3,000 new, you can get a gun like my current one, a Browning XS Special with high post rib. It has adjustable comb, and can be set up for all three games.
Danny