in each event do you have only one value (to or from or user) or could you have in the same event more fields?
If you have only one field for event, you can use coalesce function:
| eval email=coalesce(to,from,user)
If instewad you surely have all the three fields in each event, you can use a solution like the one of @harsmarvania57
| eval email=to."--".from."--".user
if you could have more fields in the same event but you're not sure that there are always all the three fields, you have to add a value to the three fields when there's no value before the eval row:
| eval to=fillnull(to,"-"), from=fillnull(from,"-"), user=fillnull(user,"-") | eval email=to."--".from."--".user
otherwise the eval command doesn't run.