There have been a number of Doctor Who anniversary crossover specials – to date, to mark various anniversaries in the shows history.
There have been 5 to date :
1) The Three Doctors : 1973 – 10th Anniversary Special
2) The Five Doctors : 1983 – 20th Anniversary Special
3) The Two Doctors -1985
4) Remembrance Of The Daleks : 1988 – 25th Anniversary episode
They have been a generally positive affair on the whole, especially the Doctor crossover episodes – Two Doctors, Three Doctors and the Five Doctors.
These stories featured different combinations of Doctor incarnations including :
1) The Three Doctors : 1973 featuring First, Second and Third Incarnations – William Hartnell, Patrick Troughton and Jon Pertwee.
2) The Five Doctors : 1983 – 20th Anniversary Special featuring First, Second, Third and Fifth Incarnations – William Hartnell, Patrick Troughton, Jon Pertwee, Tom Baker (in archive footage) and Peter Davison.
3) The Two Doctors -1985 featuring Second and Sixth Incarnations – Patrick Troughton and Colin Baker.
Crossover stories have always been problematic in that they have to plausibly circum-navigate the First Law Of The Time – which states; “….no incarnation of a Timelord, will cross timelines with any other past or future incarnation.”
The above specials found ways around this; however, including a devestating attack on Gallifrey by previous Timelord founder – Omega from an anti-matter universe. Which prompt the Timelords to call on the first Three Doc’s in The Three Doctors.
In The Five Doctors, four of the first five incarnations were successfully timescooped out of their timelines by a forbidden Timelord weapon.
In The Two Doctors, the second and sixth Doctors are brought together merely by visiting the same spacestation, during a similar time period.
Remembrance Of The Daleks wasn’t a “physical” Doctor crossover story. Though it did include a backstory linking the First and Seventh incarnations, via the hand of Omega. It also featured the use of familiar locations from The Unearthly Child.
Dimensions In Time : 1993 – was a short 30th Anniversary Children In Need sketch featuring the Third, Fourth, Fifth, Sixth and Seventh incarnations in a bizarre short featuring Eastenders and the cast of Albert Square.
The Doctor kept cycling through each of his generations, sets of companions and associated monsters. It wasn’t really canon in the sense of the series and was more for fun.
But it was interesting seeing a crossover featuring all of the ‘then’ incarnations, from number three – onwards.