Jack thanks for your comments. I will have a closer look at what my fingers are doing excactly and try too get a more consistent trill by placing the accurately. I will report back if I found something relevant.
About thrills with multiple fingers: Mr Delcamp suggest using multiple fingers because he is the opinion that this will increase the speed of the thrill, especially if there are multiple trills following each other, or so much I understand from his link all the way at the top of the page.
I never played thrills this way before. But, I do have the feeling that using multiple fingers increases the speed of the thrill. The problem for me is that I am using the 4th finger a lot more this way and this tends to slow the thrill down. For a thrill performed 1020 or 1213 I can reach a pretty good speed, or at least, I am happy with it. I dont think I could reach the same speed that easily (after a weeks practice) if I only used one finger. Increasing the x3x4 thrill will take some time for me, my pinky is sooo lazy