Can I use three #8 thhn wires in 1/2" pvc conduit about 25' to an electric stove

Well, 1/2" PVC Sch 40 is listed to hold 3#8 THHN , but you need a fourth wire, at least a #10 green THHN for the equipment ground. A new range needs two hots, 1 neutral, and 1 eq ground wire. So need to use the next size conduit,3/4" PVC.

If you are planning to use the neutral at the range to ground the unit, this method is no longer allowed.