For the years near 1920, have you checked city directories, to find the address of the family in those years (say, 1917 to 1923)? You can check to see if the address in the 1920 census is the same, or at least nearby.
Also, is the occupation of the great grandfather in the 1920 census what you would expect, based on other records that you have for him?
I've seen names and genders and occasionally ages for children mixed up. A son named Barney was mis-heard by the census-taker as 'Bonnie' and so is listed as a girl. I've also seen a child left out entirely, even though I know they were very much alive.
As for the mystery third child - it could be a child that died young, that you don't know about, though perhaps you can look for a birth or death certificate or other record for that name It may also be a relative's child that was living temporarily with the family. My mother was routinely sent to spend entire summers with her aunt in the 1930s, and I can see how she might have ended up listed as part of the aunt's family.
Lak/Lok/Liak/Lock and Kalon/Kolon in Zagare/Joniskis/Gruzdziai, Lithuania
Lak/Lok/Liak/Lock in Plunge/Telsiai in Lithuania
Trisinsky/Trushinsky/Sturisky and Leybman in Dotnuva, Lithuania
Olitsky in Alytus, Suwalki, Poland/Lithuania
Gutman/Goodman in Czestochowa, Poland
Lavine/Lev/Lew in Trenton, New Jersey and Lida/Vilna gub., Belarus