Is it effectively possible to get rid of the yellow triangle in download mode on an S3 with 4.3?
First of all, if you're not using KNOX you don't need to worry about the triangle status. Second, to use TriangleAway you have to be rooted, with SuperSU. If the Google Play version doesn't work, try the latest beta v1.99r3 which should fix many SEAndroid related issues. In fact it is highly recommended that you get rooted, so that you can remove both KNOX, OTA updates, and the triangle, before you will receive the next OTA update which may completely lock down your device. If you have never flashed your phone, and it is showing the triangle, you have either received a refurbished device or you have been cheated by the seller. The triangle is an indication that a OTP (one time programmable) fuse has been blown in the past, due to flashing.