A simple use of DEBUGGER would had told you that what was the final command text formed. All you needed was to look at that text and see if there is any issue. Just to confirm, copy it and paste the raw query formed in SQL server and see if it executes without error. This would tell you the issue with the query, if any.
Try (Looks like quotes were missing for username value):
MAcmd.CommandText = "INSERT INTO transaction (username, withdrawamount, date) VALUES ('"+textBox3.Text+"', '"+textBox4.Text+"','"+textBox1.Text+"')";
P.S.: This is assuming that the datatype for the fields being inserted is of varchar type. In case the database fields are different datatype, then you might need to format the values accordingly before inserting.
1. Always use parameterized query
]. This would avoid confusion and syntax errors that can come into picture because of quotes. Further, it takes away SQL Injection issue.
2. Always use meaningful names of the control such that you can easily understand what value that control/object hold.
Put a quote just before semicolon too. Query does not need a semicolon at the end.