I am not familiar with any bus transmission that is 3-speeds and would be hooked to a M-B engine unless the bus in question is more than 40-years old.
In as 2003 Freightliner I would expect the transmission to be an Allison 2000 or 3000 series transmission.
M-B had some real teething issues with their engines when they were put into school buses. Most of the teething issues were resolved but it meant a lot of the buses were garage queens. The newer versions didn't have nearly as many problems.
When the new emissions standards came in M-B decided it wasn't worth the effort to try and meet the standards with the engines available here. The European spe'c engines that would meet the standard would be cost prohibitive in the US market.
While Freightliner is required to service, repair, and provide parts support for M-B engines it doesn't mean they have to do it in a timely or economical fashion.
In other words, M-B engines are a sort of orphan out there and can become real $$$$ pits.