Be careful. Some 90-30 modules will not work with the Rx3i.
Look for the Rx3i systems manual (GFK-2314) on the GE community site. It has a list of 90-30 modules that will and will not work.
If you have 90-30 modules that use isolated 24Vdc, you have to supply the universal backplane (Rack 0) with 24Vdc. There is a terminal strip on the left side of the backplane. +24Vdc goes to terminal 7 and 0Vdc goes to terminal 8.
Details of the backplane are also in the GFK-2314 manual.
Expansion racks and remote racks still supply the isolated 24Vdc.