The fact that you apparently put your left pinkie in the third slide ring suggests that you have pretty small hands. I don't consider my hands to be huge (though admittedly I do buy XL gloves to get sufficient finger-length) and I don't think I *could* comfortably put my left pinkie in a fixed third-slide finger ring and play. I was taught that the "classical" grip involved putting your left ring-finger in the third-slide ring, and operating any first-slide intonation adjustment with your left thumb. My first good trumpet was a Selmer K-Modified with a first-valve trigger and third-slide ring, so I dealt with the ergonomic situation you're talking about. And I was taught to put my right thumb between the first and second valves. Somehow I managed. Honestly, the only time I ever needed to extend both first and third slides at the same time was to play a low F with all three valves. I've long-since changed my left-hand grip to the "Vulcan" style: two fingers on top of the third-valve slide, two below. This works for me, whether I'm playing my Olds Recording with a third-valve trigger or a trumpet with a third-slide ring.