O ile wiem, to ten zapis wymawia się w każdym z tych języków inaczej:
w rosyjskim szsz, ukraińskim szcz, a w bułgarskim szt.
It's basically ś (or rather śś) in standard Russian, which is always contrasted to ш (which is equivalent to Polish sz), and not by its length (which is variable). One must note, however, that in Polish ś may be alternatively pronounced as [sʲ] (in the more etymological manner), which is a separate phoneme in Russian (orthographically it's сь or, before the "iotated" vowel letters, just с: cf. лось "moose", семь "seven", сёл "of villages" on Forvo).
The most typical non-standard dialectal pronunciation in Russian is szsz, though pronunciations ść and szt have been attested as well (the former actually was the orthoepic pronunciation in Russian until the early 20th century).
In Ukrainian it's, indeed,
szcz orthoepically, but contractions to szsz are possible, especially in certain frequent words (like що). Ukrainian, however, generally treats its sibilants in a manner slightly different to Russian, Belarusian and Polish.