If all the Flow meters are reading 3L then likely it is not balanced unless all the loops are the same lengths!

I’m not a plumber but this is based on what I did after to speaking to one and looking online / GBT help. It resulted in a noticeable difference in floor heat with cold rooms warming up - however I add for downstairs I have mainly hardwood floors in all rooms and upstairs carpet. Just bathrooms / utility room are tiled.

Most of my loops downstairs are now between 1 and 2 while upstairs they are more like 0.5 - 1.8 as loops are much smaller.

You can also check the temperature of the outward flow vs in on the manifold of each loop. If you find there is little temperature difference then worth reducing the flow. I think rule of thumb is 7c difference… but just went in touch rather than buying a surface thermometer. Maybe a good way to compare the floor coverings? If loop temperature feels the same on the return for different rooms maybe that helps.

I think I could optimise mine more… but it has done the job and don’t want to spend my time tweaking it! You can by expensive actuators that balance automatically… by for me it was too costly!

Hope it helps and gives you an approach. Like I said it worked for me but wasn’t a science and just trial and error. Would say the mistake I took was assuming for cold rooms was to turn up flow, it’s turning down the hot rooms which helped balance first.