so in this timeline (undetale) sans was thinking "what if i counter the human in true lab instand in the last corridor" so sans (undertale) counter human in true lab

in other timeline (COTV) sans was test the machine if it work but sans accidently make some portal that suck him to a portal

in other timeline (UTLB) sans was waiting human but because COTV timeline is near UTLB timeline the corridor was fill with void and mysterious portal and because sans wasn"t react so fast he get suck to the portal

in undertale sans was counter human they about to fight each outher but theres a portal that theres 2 skeleton that same as sans (COTV sans and UTLB sans) because LB sans was shock to see human and he didn't perpared he stop human by using blue bone

undertale sans was confuse as why there are 2 of him

because is in true lab COTV sans quickly turn on the machine once again and it works

now the human have to fight 3 sans's at once