Richard
You can choose any 3 from 173.8, 174.1, 174.5 and 174.8 and add 175.0 for intermod free operation. External noise is a separate problem which is not so easily overcome. I haven't looked at the new frequencies as my S3500 kit hasn't got them loaded. A quick look says that they are a disaster waiting to happen with 200KHz spacing all the way.
The old 5 channels give 300, 400, 300, 200 KHz spacings so you only require to loose 1 channel from the first 4 to be intermod free.
All 5 of the old channels are intermod free of 3rd and 5th order products but only for 2 transmitters in close proximity
The 4 channel scenario works for 3 transmitters, and is generally considered best practice.
Brian