When I installed my gates I consulted the online Manual of Uniform Traffic Control Devices articles for protypical guidence. According to the MUTCD, protection is to be provided where there is at least a car length or more space between tracks. So in your picture, you would need three sets of gates or flashers or crossbucks. If all three tracks were adjacent without intervening roadway, then one pair of protection, is required.
And gates are to be set at least 10 or more O scale feet (about 2.5 inches) perpendicular with roadway from the closest edge of track (in your case where the track curves.) to prevent train from hitting gates. And then you have to place a white line on roadway about 10-15 feet before protection devices to denote where traffic is to stop.
More info than you wanted, but of course its your RR so you can model it as you want.