How do you write a multiple IF formula

You can nest IF functions like:

=IF(condition1, if_1_true, if_2_false)

Then second condition goes:

IF(condition1, IF(condition2, if_2_true, if_2_false), if_1_false)...

And so on....

So in your case:

=IF(A3<B3, B5+B6, IF(A3=B3, B5*B7, 0))