Civ V - Longbowman doesn't have range 3 or strength 18

The range of 2 is correct. Longbowman have a base of 2 range but they also start with the promotion which gives +1 range. The result is mostly the same- they should be able to shoot 3 tiles away.

If they had 3 range base, they would be able to take the range promotion on top of that for 4 range- which would be a bit silly. On the plus side, it means they upgrade into Gatling Guns with extra range (if you have Gods & Kings).

The 15 strength is also correct in Vanilla. The Gods & Kings expansion changes the combat system and, for balance, a lot of the unit strength values. Annoyingly, the wiki doesn't mention this. Crossbowmen and their unique versions only have 15 strength in vanilla.