If your PH is coming out lower than when its going in then its probably picking some some acidic salts on the way out. Wouldn't hurt to give it a good flush.if your roots are looking good then that should do the trick.
How often you water depends on how fast your medium drains, how much air circulation moves through it, how much water and air are retained by the medium, and how large your planters are. Not to mention how large the plant is, how much air circulation there is around the plants, and the humidity. All these affect evaporation and how much water the plants take in.
Your mix of 70/30 is not nearly as aerated as mine (only about 40% coco) so you probably only need to water once every 2-3 days, But when they get bigger, with more large leaves, since your in a 3 gallon planter you'll likely need to increase the amount you water. Plants with lots of large fan leaves will use more water too. In your case I'd start by letting the top inch of medium dry and then use the "lift the pot method". I use the "lift the pot method" to decide if I need to start increasing the watering schedule.
What I've found out working in coco is that the aeration in the medium is crucial_ you need to water accordingly. A more dense medium like the 70/30 your using is going to drain slower and hold more water than the aerated medium I'm using. The more fine particulate matter also absorbs more water and after you've been watering it for a while those fine particles settle down through the medium to the bottom of the planter, which also slows down how fast they drain. Ever wonder why your perlite "works its way to the surface"?
Its really not...the fine particulates are just settling and since the perlite is course and very light in weight it doesn't_ so it seems that its working its way to the surface when its actually just sitting there.
This isn't to say that you need to change your medium_ you just need to find out what works best for what your using. It took me about 6 coco grows until I settled on a mix I liked best.
And when you flush you need to run a lot of water through those planters. If you're only giving them a few cups then try running a gallon or so through one and see how much runoff you get... wait an hour and if your plant isn't dropping run another gallon through. And so on. Once you've run 3 gallons through they should be pretty well stabilized. Once you get more than 30% runoff your coco has absorbed just about all the water it can and the planter will be as heavy as its going to get. Use that as a base for judging how light it gets after a few days of no watering.
Cheers!