Inability to secure pick

I cannot keep the pick from moving out of my grip whenever I try to speed up when tremolo picking. It can occur while strumming and doing other forms of picking, but for simplicity, the tremolo displays the problem best and it seems to happen more quickly the faster I get. There have been times when I thought I fixed it and it wasn't an issue, but unfortunately I didn't know what I was suddenly doing right which means I have to figure out what I'm doing wrong if I'm going to get consistent. I'm trying a three finger grip because I can utilize the "tall-mouse" movement more readily for greater speed, but whether I use three or two fingers, it's the same issue.