The underutilized amount is equal to zero since they were not eligible for the plan before, so the catch-up is limited to that, which is zero.
Now 2021 is over. If they entered in 2021 and no allocation was done for 2021, then they have an underutilized amount now equal to $19,500 which can be used now.
Also, read the 3-year language carefully, my understanding is that the year they reach normal retirement age is not a year that can provide a catch-up, it’s the 3 years prior to the year they reach normal retirement, but check the document.