While there are a few grandfathering exceptions, metal box or plastic, if you have a ground wire you should use a switch with a ground screw and connect the ground wire to the switch. Note that not all three-way switches have the terminals in the same location so be careful when connecting up the new one to get the common wire to the correct screw.

Putting a high impedance meter (such as yours) on an unterminated wire that's running in a house with other power present will induce stray voltages. That's what your 30V almost certainly is. It will not be a problem.

An aside for those who'll jump all over me for glossing over the possible options, zver11 you can skip reading what follows:

Grounding a switch through the metal ears is only permitted when the box provides that... which even some plastic boxes do but you don't seem to see those south of the Canadian border... and the switch is listed to be grounded that way... it will typically have spring contacts on the mounting ears.