Peter Davison, Colin Baker and Sylvestor McCoy did indeed appear in The Day of the Doctor, but not necessarily in person.
The answer by onigame presents the conflicting evidence on whether they were under the sheets in the scene mentioned in the question (Steven Moffat says they were not, Peter Davison says they were). Considering that The Five(ish) Doctors Reboot is a spoof about the actors who previously played the doctor trying and failing to get in on the anniversary episode, I'm inclined to think that it is Davison who is joking. But everyone is free to decide for themselves.
The scene where they definitely appear is, of course, the one where the 13 TARDISs surround Gallifrey during the Daleks' final attack. We see a brief image of each Doctor as he contacts Gallifrey's War Council. The images and audio were not recorded for the anniversary special, but were taken from those Doctors' original episodes.
I haven't verified this, but The TARDIS Wikia states that the footage came from
- Fifth Doctor: Frontios with audio from The Five Doctors
- Sixth Doctor: Attack of the Cybermen
- Seventh Doctor: Battlefield and Doctor Who (the 1996 movie)