On the question of whether Sigurd married a 3rd daughter of Malcolm II, or whether he married Donalda before the death of her first husband, Findlaech in 1020:
1) Project Medlands describes it so:
SIGURD "Digri/the Stout", son of [HLODVIR Torfinnsson Jarl of Orkney & his wife Eithne of Ireland] ([965/70]-killed in battle Clontarf 23 Apr 1014).
m ([1005 or after]) --- of Scotland, daughter of MALCOLM II King of Scotland & his wife ---. Orkneyinga Saga records that “Earl Sigurd” married “the daughter of Malcolm King of Scots” (whose succession is recorded in 1005)[240]. Snorre records the marriage of "Sigurd the Thick" and "a daughter of the Scottish king Malcolm"[241]. It appears unlikely that Sigurd´s wife could have been King Malcolm´s daughter Donada (as shown in many secondary sources, including the Complete Peerage[242]) if it is correct that Donada´s recorded husband Findlaech was killed in 1020 and that their son was born in [1005][243].