It's very dependant on your situation and which army you are facing. But like ComputerHeat said, 3 units is too few, you will get into trouble quickly and lose map control.
Against the USF you usually expect to see a fast M20 or AA halftrack so it's generally a good idea to get a PaK out as soon as you have a decent army composition (4-5 units from T1).
Against soviets you don't generally need a PaK so early since the T3 vehicles come out significantly later than the USF light vehicles. Against soviets you can get a squad of panzergrenadiers or a scout car followed by a PaK when needed.
As a rule of thumb you don't want to stay in T1 too long unless you have a good map advantage and you're going to skip T2 and go straight for Pz IV or something. Usually you should have T2 building up after 5-6 minutes. However if you're staying longer in T1 it's a good idea to get more units out of it to hold your part of the map - an extra MG or grenadier etc.
Generally against infantry swarms the MG42 is good but it takes some experience to make good use of it. It gets very easily flanked on some maps if you make one mistake and usually you need to constantly reposition it to keep your opponent guessing. Also the mortar is a very strong unit and it helps keeping those infantry swarms away if you can hold the line.
Just some very general thoughts and very quickly written, hope it helps