Tribu/tribù/tribo and ímpetu/impeto seem to be "cultismos" in all the romance languages (including the regulars form in -o). But the -u in espíritu is not so obvious to me for the following reasons:
1. The word “spiritus” (whatever was it’s spelled form in the past) necesseraliy exsisted in Spanish "continuousely", not only from 1440. With other words, it cannot be a "new" term, existing only from the 15th century.
2. In the Glosas Emilianenses (around 1000 Anno Domini) we find:
... e qual dueño tienet era (=ela?) mandacione cono Patre cono Spiritu Sancto, enos sieculos de los sieculos ...
Yes, even with o they are only partially adapted, but I'm not sure what that would mean in Italian, that evolved differently perhaps less, not dropping unaccented syllables, changing consonants etc.
I think it means when Espíritu started to be used naturally as a genuine word in Spanish (genio), for example espíritu santo, and not just a Latin quote. Of course, Spiritus and probably all other religious terms from Latin existed constantly from the start. Ela > La
Yes, it did have time to change to /o/ as it developed epenthetical e- but perhaps it has something to do with it being a cultured word, not at all common in daily speech? Also what I said before that changes are not usually retroactive. U became o quite early, but it isn't cut and dry. Lots of (short lived) Latinisms during the baroque time were brought into the language with -o. Perhaps the church also had an influence... using latin?
Besides, all 3 words we have mentioned are still pretty cultivated words in Spanish. For example, Esprit (adapted) has taken on some very colloquial meanings in French that Espíritu doesn't have in Spanish. Perhaps in Italian too, I know there's "spirtoso". Just a thought that might contribute to the -u...
Edit: Ok, Latin Spíritus is dated 1220-50, but still later than your Glosas Emilianenses...