I have a degee in chem. Your intuition is spot on. Lets assume Venera 3 wasn't sterilized (reports in 1966 say it was [1] but the effectiveness is contested [2]).

Even if spores did survive the space trip (actually quite plausible, [0], venus trip is ~110 days), the atmosphere is extremely dessicating and acidic (SO2, SO3, H2SO4), and the ground is too hot for any known life. The dessication and osmotic pressure is the real kicker. It's very unlikely spores would germinate without rehydration.

[0] https://www.nasa.gov/mission_pages/station/research/news/eu_...

[1] https://www.nytimes.com/1966/03/06/archives/soviet-says-venu...

[2] https://www.jstor.org/stable/1720963