Part of what makes some cards good is that it only affects one unit, like Airlift. I think it would significantly affect balance if you could use Airlift 2 in place of Airlift to move one unit.
I'm ok with the way it works now, but I'd like to see more new cards that give buffs/debuffs to multiple units to save card space.
I wouldn't apply my comment to airlift card. Obviously there is "octane" and "high octane" which affect just one unit. But unlike the "evac" and "airlift series", this card is not called "octane 3" or "airlift three." The name, "We Need Reinforcements" gives no indication that it is a part of a series where there is a one, two, and three version like these others. Also, "We Need Reinforcements" is a simple buff card. The price you pay if you were to use it on one unit instead of three is that you are paying 40g when you would be much better off having a "high octane" card - not to mention high octane gives 100% speed bonus compared to 75% speed bonus.
Overall, I get the point though. It would change the card. But is the critical downside of "We Need Reinforcements" that you need three mechanical units? Would the card become too powerful if it could be used on less units? I never felt that requiring that it apply to three units instead of "three or less units" was a crucial element. The downside of "We Need Reinforcements" is the cost.
I think in this individual instance the annoyance outweighs the need for restriction.