no.

but you might be able to do something like;

instance pair 1 mirroring on port 5023

instance pair 2 mirroring on port 5024

instance pair 3 mirroring on port 5025

the witness instance has 1 endpoint all on port 5022

on each of the principal servers, run;

ALTER DATABASE <DATABASE_NAME>

SET WITNESS =

' TCP :// < system-address of the witness server > : 5022'

GO

however, the seperate instance on the witness will lead to a better solution imho....