Yes you can join 3/16 to the mainline as long as you keep the 2% grade. Just use saddles to join each 3/16 to the mainline, do not join them together before entering the mainline. There are many different saddles available out there, but since I first tried the DSD saddle I've used no others. As others in my system fail, the get changed to the DSD.
With the DSD, just be careful not to over tighten the SS band clamp, it will actually squeeze a waist into the mainline. If one needs tweaking, just give the clamp a slight turn on the screw.
For the mainline on gravity, be certain you have no dips. If necessary you may in places need to add some posts to support the line. Another item I really like from DSD is their stubby mainline drill bit. Just chuck it in a cordless drill and drill the hole for a saddle in the line, it is short enough so it will not go thru the opposite side and the point is good enough that you need nothing to hold the bit from dancing as you start to drill. I've only gotten them from DSD, others might of started making them too by now, I do not know. If you never use that bit on anything other than drilling mainline it may well last 20, 30 or more years.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.