Is it possible to log a 3 wire pressure sensor to the flash scan v2 without the sensor being used by anything else?

When you say 3 wire pressure sensor do you mean?
1) Supply
2) GND
3) Output

If so yes, you can use one of the V2's analog inputs, however, since the V2 is not a power supply you must figure out you own means of supplying power to the sensor. You will also need the output specs from the manufacture (unless you figure it out yourself) so you can build a PID.

can i use the pcm for the 5v supply without any adverse effects on my flashscan?

I would be more worried about adverse effects on your ECM....I guess it would largely depend on the currrent required by your sensor. I use the LMA-3 (Aux Box) from Innovate Motorsports or you could build your own supply.

http://www.innovatemotorsports.com/products/lma3.php

You can get a 5 volt linear regulator really cheap, then use that to run the sensor.