18 AWG wire is rated for 7A. For 3.5A you will need something like 20 or 22 AWG wire. You might! I repeat. You Might be able to use 24 AWG wire, but I am not sure. I don't have my handy little chart in front of me right now to verify that 24 AWG is rated for 3.5 A.
802.3bt Type 3 will give you about 51W (60W burst), with your LCD needing 42. Should be OK but I would test it.
Type 4 will get you 71w but I have not seen a switch supporting it yet.