I've been to all three cities, but it's maaany years ago since I was in Khabarovsk. It has probably changed a lot since then. Summers were unbearably hot and humid, with hoards of insects. Winter temperatures seldom go below 20-25, but the wind chill can make it feel much colder. Spring can be very dirty when all the snow melts. The best periods are probably late spring or fall, although I saw lots of nice ice scupltures there in winter.
To me as a European, it's much more exotic than e.g. Moscow. There are lots of Chinese and Koreans in the city, and there are Chinese markets. I was mostly working so had little time for sightseeing.