Hi Marc, and welcome to the forum!
Great to hear that you have fun playing the game with your wife and son!
Regarding team mode games with three players: The main problem is that the (maximum) number of players is defined in the level itself (by the level designer). For example, most Emerald Mine levels were designed to be playable by one or two players (so they contain two players, where the second player is removed when playing in single-player mode).
There are three possible solutions (some of them practical, some theoretical):
1. Look out for level sets with more than two players. Unfortunately, I'm afraid than there are virtually none of them out there (with the possible exception of a level set by SlappyHappy2000, who has indeed created some levels for three and four players and who has released them here in the forum a few weeks ago, if I remember right -- I'll check for the link).
2. Modify existing level sets to be playable with more than two players. This works like this: Copy an existing level set directory (say, "levels/Emerald_Mine_Club/emc_ace_mine_1") to a new directory (say, "levels/emc_ace_mine_1_for_3_players"). You might like to also change the name of that level set (by editing file "levels/emc_ace_mine_1_for_3_players/levelinfo.conf" for our example level set, changing "name: Ace Mine 1" to "name: Ace Mine 1 for 3 players", for example). Then open the levels of your copied level set in the level editor and just add one or two players -- don't forget to also add an additional exit for each new player! Et voila -- you can now play that level with three or four players! (Please note that this might not work with all levels.)
3. A future version of R'n'D might contain a new functionality to automatically add players (and exits) to existing levels. This would always be an experimental feature, which might result in unplayable/unsolvable levels. But it wouldn't be too difficult to add such a feature, I think.
I hope that I was able to help a little bit -- especially solution 2. might be useful and practical in your case!
Have fun!
![Smile :-)](./images/smilies/icon_smile.gif)