If a Siamese twin was born exept with three people instead of two would they be considered triplets?

They would be called conjoined triplets. Siamese is not a politically correct term anymore.

This has happened though.

All conjoined - I could only find two cases.

Unidentified (Sicily, Italy, 1834) 3 boys born with a single torso, two hearts, two stomachs, two lungs & three heads. The case was profiled in Gould & Pyle's CuriOSities of Medicine.

Unidentified (Samsun, Turkey, 1955?) Three heads, two pairs of arms, two pairs of legs; lived for 2 hours. On autopsy four lungs, three livers, three brains, two hearts and two kidneys were found. Information appeared in Sexology Magazine in 1955, they cited the Journal of the American Medical Association for the information.

There have been 43 sets of triplets born with 2 conjoined. Only 6 of those sets have all three surviving, 17 sets I have no stats on, and the rest had one or both conjoined babies die.