idk honestly. i'm sure my T is very very low because it was low before I even started HRT (around 320 ng/dL, with the average male level being 680 ng/dL - by comparison women's levels are usually around 15-70 ng/dL). for the first couple months i was dysfunctional in the sense that i had no sex drive whatsoever, but i could still get and maintain an erection with ease and i did so every couple of weeks. trans girls can usually maintain a functional erection if they do it every once in a while (think of it as "use it or lose it", because if you dont use it it will atrophy). my sex drive has now recovered and i find myself having zero issue performing, but things are 'different' now for sure lmao.
as for you, 3 days on spironolactone is not long enough to cause a serious drop in testosterone. in fact usually for the first little while, there is a spike in testosterone before it starts dipping. it takes months for people like me to get our T fully suppressed, so 3 days on spironolactone is like literally nothing in the grand scheme of things. if i start psyching myself out about whether or not i will be able to get an erection/perform, my erection/performance suffers. this was true even pre-hormones. (tl;dr version - i doubt its the spironolactone. worry less, if that doesnt help talk to your doctor lol)
idk anything about topical estrogen but if i had to speculate based on my limited knowledge of how all this works, i suspect it would not work as well as you're hoping and it has a pretty high chance of going systemic. i think the main enemy is androgens (both dht AND t, i think that T's role in male pattern baldness is severely underestimated!), which estrogen doesnt necessarily combat (which is why I have to take antiandrogens in combination with my estrogen). I've also heard of some trans girls using creams as part of their hrt (but oral pills/injections are the preferred method) so obviously it must go systemic if its being used for that purpose. not sure how/if testosterone may relate to this, but they make creams for that too and it definitely goes systemic. source: my trans guy friend uses something called androgel (i think?), but he's moving to injections soon as they usually offer better results.