If you can't put the "and" part as a sub-event to the "or" part, You can use a boolean trick.
the following is: If This1="a" or (This2="a" and This3="a")
Under system, use the event "Compare two values"
(This1="a") + ((This2="a") * (This3="a"))
the comparison is "Greater than or Equal"
value to compare to is 1
The equation in parenthesis is a boolean value, if true then the value is 1 which is what "true" equals, if it were false, the value is 0.
So if the first field, (This1="a"), is true, it will be 1.
If both the other are true, they both equal 1, so 1 times 1 is 1 as well. If either is false, then 1 times 0 will be 0.
both results are added together so anything over 1 means the statement found a positive result and performs the actions.
Does that do what you need?