I have heard that we may add our own specific requests in the middle set of brachot of the Amida, if we feel the need, but that additions are not allowed in the first 3 or last 3 brachot. Why are we not allowed to add our own specific thanks in the last 3 brachot?
I saw this here
We have written (Living the Halachic Process, vol. II, A-1) that it is permitted and positive to thank Hashem but that it can be problematic to do so in a manner that makes it appear like a set beracha. Attaching appropriate statements to an existing beracha, on the one hand, makes it unlikely for there to be an issue of beracha l’vatatala (see Mishna Berura 108:38). On the other hand, changes in the content can be more problematic (See Berachot 40b; Shut HaRambam 254).
And the next paragraph:
Since the lack of sources and apparent minhag is to not add personal thanks during Modim, then even though it is probably halachically permitted, we do not suggest it when other options exist. Before Elokai Netzor and during Shema Koleinu, one can say anything appropriate, and while that is classically for requests, it should be fine to thank Hashem as well. Realize that even the middle, “request” berachot include praise.