All the well-known brands are OK. Many people, including me, like Garvo if you happen to live near a stockist. Dodson & Horrel do a non-GM version, if GM ingredients bother you. Another reliable brand is Marriages. They do layers pellets with Flubenvet for when the girls need worming, but with only 3 you'd be better off buying Flubenvet powder and adding it to your usual pellets, as even the 10kilo bag of treated pellets would be far too much for one treatment and would be out of date 6 months later when you wanted to repeat the course. Do watch the date when you buy pellets, as with only 3 birds a 20kilo bag will last you nearly 3 months I expect, and especially in hot weather (what hot weather?) the vitamins etc may deteriorate with time.
Not too sure about the grass clippings - I've never given them to mine, though as you say, they love them, because I've read that they can cause crop blockages, maybe someone else can advise on this. Apart from that, your feeding sounds fine - a good simple mixed diet and plenty of fresh water is really all they need, and as long as the pellets are in date and kept in a sealed container, away from vermin and damp, probably all of them are satisfactory. However, I expect the differences in quality of ingredients may well be reflected in the price, and you may find that some of the cheaper ones are actually less economic because the girls don't do so well on them, or actually eat more to keep themselves going. It really depends on what's available locally, assuming you can collect yourself - paying for transport becomes very expensive since the price of feed itself has gone up so much in the past year or so.
