That would be incorrect......
As long as you keep the rating valid, the "differences training" has no expiry date.......Thats why its differences training, not additional type rating...
Why would I go to the expense of keeping an EC155 type rating current for 3 years if I was flying an AW139 just in case one day there might be a job advert for an EC155 pilot that I would be interested in? In more enlightened parts of the world it would be recognised that (as I had a factory EC155 type rating and years of EC155 flying experience) that I might just not need to complete a full type rating course again and a simple check flight with a company TRE might be adequate.