Working on converting an old CB to a 10 meter beacon. Obtained some
xtals in the correct range I need, but they have 3 leads instead of 2.
Are these resonators? Can I use them as direct replacement for xtals in
the CB radio? Where does the 3rd lead go?

Do a search for 21.4 MHz filters. They have three leads and are used as
bandpass filters in UHF CB radios and may also turn up in 27MHz radios.
The middle lead probably goes to ground.
Alan