really so far we have had 3 regenerations
Well, er... no.
We've had the first Doctor "rejuvenation" into the second Doctor because his body "wore a bit thin".
The second Doctor being forced by the Time Lords to choose the appearance of his next regeneration (why he wanted to look like Jon Pertwee is anyone's guess).
The third Doctor fading into the fourth.
The fourth Doctor turning into a strange being known as the Watcher, himself turning into the fifth Doctor.
The fifth having hallucinations before turning into the sixth.
The sixth regenerating into the seventh off screen.
The seventh morphing into the eighth.
Romana trying on different bodies between regenerations.