The SX3 is an auto-loader so I doubt the pins are in the same location. The SXP is basically a modernized Win 1300 and is a clone to the FNH P12. I have been playing with these a bunch lately and just did a class for a department that switched from the 870s to the SXPs. They were very happy with the choice.
Lights on the mag tube are simple. Integrated into the fore end, a little tougher. I used a CTC Railmaster 202 (white light) on a tube mounted pic rail and it has been working so far. There will be more aftermarket stuff for them soon.
Good Shooting, MarkCO
.40 S&W Club #1, 1911 Club #1067
NRA Life Member and Certified Instructor