What is largest BCD encoded decimal value that can be represented in three bytes?

A BCD digit only uses the binary patterns that represent decimal numbers, ie 0000 - 1001; this requires 4 bits (1 nybble) so there can be 2 BCD digits to a byte. Therefore in 3 bytes there can be 3 × 2 = 6 BCD digits. The largest BCD digit is 1001 = 9.

Assuming non-signed, the maximum 3 byte BCD number is 999,999.