my uncle uses straight talk and needs a new phone, but my pixel 3 is under verizon is it possible to just put his sim card in and it work as his phone now.
It should work, since the VZ Pixel 3 is supposed to unlock within 24 hours after first use on VZ. A test with your uncle's sim would confirm. You'll want to test MMS functionality. You'll need to make sure the APN settings are configured properly for ST. Their website walks you through the process.
Straight Talk isn't a carrier, they're an MVNO, so they use the big 4 carriers. If you get a Straight Talk SIM for Verizon, it will work. But it's SIM-locked, so if he wants to use a different carrier (the other choices are AT&T, Sprint and TMobile), you'll have to get it unlocked by Verizon
Put his SIM in the phone, If it asks for an unlock code, see Verizon's unlock page. If it doesn't ask for an unlock code, it's working (and it's not SIM-locked).