Seven Keys to Doomsday, by Terrance Dicks, has the Doctor acquiring two new companions, regenerating into Trevor Martin, and then going on a rather pointless quest to find seven bits of crystal which fit together to form a mystical key. It obviously inspired the Key to Time season a few years later, but I think it looked better than it sounds.
The Ultimate Adventure, also by Terrance Dicks, has Colin Baker, a French companion, and several songs, as at the request of Prime Minister Margaret Thatcher they try and rescue the American delegate to the world peace conference who has been captured by Daleks and Cybermen. Unlike Seven Keys to Doomsday, it is not actually bad, but it is on the whimsical side.
The best of the three plays is the earliest, Curse of the Daleks, written in 1965 by Terry Nation. I was amused to note that we begin with convicts on a spaceship which they manage to take control of and bring to another planet where they have to do a deal with the local baddies - the second episode of Blake's Seven reworks this with surprisingly few changes. Later parts of the plot, with the Daleks' human stooge reviving them, were recycled by David Whitaker in the first Troughton story, Power of the Daleks. On top of providing the source material for those two excellent later pieces, Curse of the Daleks has a rather good "who's the traitor" plot, and Nick Briggs does the best linking narration for any of the three plays. Recommended.