Hi annd welcome to the forum

Try this ARRAY formula...
=SUM(IF(B2:B5-A2:A5>3,C2:C5,0))
...confirmed by pressing CTRL+SHIFT+ENTER to activate the array, not just ENTER. You will know the array is active when you see curly braces { } appear around your formula. If you do not CTRL+SHIFT+ENTER you will get an error or a clearly incorrect answer. Press F2 on that cell and try again.