How to create an autonumbering formula based on three columns

Joh_Loh If your data is formatted as a structured Excel table, one possible calculated column formula could be:

=LET(
    ua, UNIQUE([Make]),
    ub, UNIQUE(Table1[[Make]:[Model]]),
    uc, UNIQUE(Table1[[Make]:[Colour]]),
    TEXTJOIN(".",,
        XMATCH([@Make], ua),
        XMATCH([@Model], FILTER(CHOOSECOLS(ub, 2), CHOOSECOLS(ub, 1)=[@Make])),
        XMATCH([@Colour], FILTER(CHOOSECOLS(uc, 3), (CHOOSECOLS(uc, 1)=[@Make])*(CHOOSECOLS(uc, 2)=[@Model])))
    )
)

As a dynamic array variant, with the MAP function:

=LET(
    ua, UNIQUE(A2:A14),
    ub, UNIQUE(A2:B14),
    uc, UNIQUE(A2:C14),
    MAP(A2:A14, B2:B14, C2:C14,
        LAMBDA(a,b,c,
            TEXTJOIN(".",,
                XMATCH(a, ua),
                XMATCH(b, FILTER(CHOOSECOLS(ub, 2), CHOOSECOLS(ub, 1)=a)),
                XMATCH(c, FILTER(CHOOSECOLS(uc, 3), (CHOOSECOLS(uc, 1)=a)*(CHOOSECOLS(uc, 2)=b)))
            )
        )
    )
)

See attached sample workbook, if necessary...