Hi
Odds are it's going to be 12 volts, working on two of the wires, try it. The third wire may well be 'live' when the solenoid is being powered, to feed another item / circuit, using an internal contact. Sending power elsewhere, perhaps to reset a valve lifter for the next start or light a warning lamp.
Go through the wires on 12 volts - it's unlikely to be less - using a mutimeter or a bulb and two bits of wire, and establish what does what. If the solenoid fails to work or is sluggish, try 24 volts whilst watching for excess warmth and the release of the 'magic smoke'.
It's a bit hard to trouble shoot for you without being there, knowing the past history of your setup, or having a wiring diagram from it's original build. Behave like a doctor and 'Do no harm'
Should you cock it up you'll have paid what Ed Dee refers to as 'School Fees'. Get another solenoid from Ebay and start afresh, they're not expensive.
Whilst typing this BruceM posted, nice to know that someone who's electrical knowledge is far greater than mine is thinking on similar lines!
Cheers
Stef