The NUON version of Iron Soldier is part 3 of the IS series. The Jag only ever got 1 & 2. And yes, the PS also got Iron Soldier 3. Graphically IS3 is a bit better on the NUON than the PS, but both versions are pretty close. The Jag is the only place where you can play IS1 or 2. Iron Soldier 2 actually came out on both the Jag as a cart and for the Jag CD, as a CD.. The cart version is much harder to find and is worth over $100. The CD version is no slouch, as it commands about $60 for a copy. Iron Soldier 3 on the NUON was actually pulled from the shelves shortly after it was released because of a critical bug found, so it is worth big money. $150+
I had IS3 for the Playstation, and didn't care for it too much. It does have a super long and awesome intro though. I also prefer IS1 over IS2, as part 2 is extremely difficult even on the easy setting! IS1's difficulty is more balanced.
I echo this sentiment, and am proud to be a member here at Retro Mags. This is truly an awesome website for us video game magazine fanatics!