I actually meant 3+1 and 1+3 as a term of how many weapons a squad gets (3xG43 as opposed to 1xLMG).
I'll sum the rest up so that we don't loose ourselves in too many details.:
The number of weapons a squad gets determines how much thought you need to put into closing in. If all models are upgraded, each lost model hurts more. That's why most SMG (/close range in general) squads are fully kitted or at least have a large number of weapons: So that you are forced to think about when to close in or not. Those units usually get some type of defensive ability like a smoke or camo. If most of your damage is concentrated on few models, then there is less thought necessary for choosing the moment to push. Those units have to work via "brute force" and generally do not get defensive abilities (Rifles, Volks, Rangers).
Another general design point is long range units having concentrated DPS so that it is harder to take them out of a fortified position by slugging it out.
The number of weapons and the number of high value and low value models is therefore more a function of "how do I need/want to use the squad", so design and balance. Not of "unit A has 2 upgraded weapons, unit B also needs 2 weapons".
There is no reason for G43 Grens to have exactly 3 G43s. But there is also no inherent reason for them not to. It depends on how we want to see the squad designed and balanced. But this has nothing to do with "squad X also gets two weapons with the upgrade". If we can create a balanced 3xG43 Grenadier squad, there is no reason not to. It would at least add something a bit more special to the upgrade compared to every other squad that uses either 2 or 4 weapons.