An option other than the one suggested by rrovers is to use the rex command:
| makeresults
| eval id="a,t1023,t456,t0123"
| makemv delim="," id
| mvexpand id
| fields - _time
| rex field=id "(?<expected_id>\w{1,3}$)"
This option adds a bit more flexibility given that you can do some validation on the content by changing the regex (e.g. you want only the id's digits just swap \w with \d in the rex command)
| makeresults
| eval ID="t1023"
| eval IDlen=len(ID)
| eval IDlen2=IDlen-2
| eval IDtest=substr(ID,IDlen2)
| table IDtest