Notice I've replaced + separator with &
Also modified the handling of spaces around the middle name so you don't get two spaces if no middle name exists
The & is the standard VBA concatenation operator and Access also uses it in queries. The + operator does double duty as both an arithmetic and a concatenation operator but it works differently from the & in how it handles nulls. You can use this difference to your advantage when you are concatenating multiple strings such as First middle and last names. If you simply use:
First & " " & Middle & " " & Last
You will end up with two spaces between first and last whenever a person has no middle name. Using the expression shown by arnel which uses both + and & you can avoid introducing the extra spaces. Examine this statement closely to understand how it works. This is much more complicated to accomplish in other languages where + is the only option. You have to use a much more complicated statement with the IIf() to avoid the extra spaces.