Macro To Validate Contents Of Cell And If Not Match Send Msg Box

Userform that people use. I need validating the users input into the form.

1. Need to validate that A1 only has 3 letters (Alpha)
2. Need to validate that A2 is not left blank
3. Need to validate A3 has only 10 digits

My state is that when the user hits the submit button in the user form the macro checks for these validations above and if everything is ok emails the sheet. However if the user havent inputed the data correctly a msg box will pop to let them know what is wrong. If thats the case i do not want the sheet emailed. below is the email code i'm using

HTML Code: 

Sub SendMail()
' Copy the sheet(1)
ThisWorkbook.Sheets(1).Copy
' Send Email
.SendMail Recipients:=Array("email.address@email.com"), Subject:="Test" & Format(Date, "dd/mmm/yy")
.Close SaveChanges:=False
End With
End Sub