need to get into programming on 3 mitel sx 200,i believe i use maintenance serial port on the back for access,all 3 of these have no clue what the passwords are,previous company went out of business that was serviceing their equipment,any help much appreciated,thanks

If they are the SX200EL/ML systems, yes the maintenance port is on the back of the main cabinet.
As for the passwords, if they're not set to default, the path of least resistance is to try all possible ways to get in touch with the person or company that knows them.
Otherwise, get in touch with a Mitel dealer who can lead you down the path of getting Mitel Inc. involved, so they can reset the passwords to default.