If I understand correctly, last year you had both a hot air oil furnace and the boiler, with the boiler running an airhandler & some some radiant, and now you've added another air handler where the oil-burner had been?
At 140F entering water temp, each of the air handlers pulls 53KBTU/hr out of the boiler for a total of 106K plus whatever the radiant is pulling, whereas last year with only one air handler you had 53K + radiant zone. The rated output of the 175 is 134K, so last year you were probably only pulling half the rated output, and now you're knocking on the limits pretty hard when all 3 zones calling for heat at once. A 53K step in load is a pretty big chunk, especially if it's taking you close to the full-fire rated output. The system behavior is probably about what you should expect now that the heat loads are higher, and not a flow problem. You could test the combustion efficiency with an analyzer, but it's probably working just fine.
During periods of lower heat load the zone calls probably weren't overlapping as often. Starting up the radiant zone cold it would be pulling far more heat for the first several minutes as it heats up the thermal mass of the water in the loops, and you're just plain pulling more than the boiler can deliver if you start them all up cold all at once. If the system is still able to satisfy the thermostats, I wouldn't sweat it if it's output is running in the 130s rather than dead-nuts-on 140F with all zones calling for heat at the same time. If they're pulling 53K per @ 140F, they're still pulling more than 45K per @ 135F.
Your true heat load is probably under 100K at design condition even with the oddball tall rooms, etc., but without a heat loss calc or a season's worth of fuel-use data to apply against weather data it's tough to say for sure what it is.
3/4" line on the propane is fine- it'll deliver well over the 175K input on such a short run.