Please see the data below

a b c
000000080672 EI0G18 1.00
EIVH38 1.00
EIVK38 1.00
000000082585 EIRH69 1.00
EIRK69 1.00

the first 3 items in column B belongs to the first item in column a.
Accordingly the next 2 items in col "b" belongs to the next item in col "A".
I want to number the first item in col "A" as 1 and the 3 items in col "B" as
1.1, 1.2 1.3 and accordingly, the next item in col "a" as number 2 and
accordingly the item 4 and 5 in col "b" as 2.1 & 2.2 and so on.....