Hi Khanh,
If I understand you well, you should use drop-down list form control on your form and chose 'SQL Query' as datasource. SQL Query should be something like this:
SELECT UserID, CONCAT(FirstName,' - ',LastName,' - ',Email) AS Test
FROM CMS_User
Also, it is important that in fields bellow (for 'Item transformation' and 'Value column') use proper values, like this {% UserID %} or {% Test %}.
In your case, I will assume that all column names and tables are correct, so you should just add correct values.
Best regards,
Dragoljub