And now - just for science 😉
If all fields are numbers and always have the same length (3/3/4):
=(A1*(10^7)) + (B1 * (10^4)) + C1Apply a cell data format of 000\.000\.0000 or use the TEXT function
=TEXT((A1*(10^7)) + (B1 * (10^4)) + C1; "000\.000\.0000")