SQL Server Index: One new table joining to three old tables, how to? how can I index this?  In a legacy system, I need to store (new table) and search on three possible natural key combos.

I'm thinking my new table will be:

declare @TableA table (pk int identity(1,1) primary key 
                      --.more columns
                      ,fk1    char(4) not null
                      ,fk2    char(2) not null
                      ,fk3    char(3) not null
                      ,fk_a1  char(5) null
                      ,fk_a2  char(2) null
                      ,fk_b1  char(6) null
                      ,fk_c1  char(1)
                      --more columns...


this new table needs to join to three different tables on these columns:

LegacyA: fk1, fk2, fk3, fk_a1, fk_a2
LegacyB: fk1, fk2, fk3, fk_b1
LegacyC: fk1, fk2, fk3, fk_c1


I'm thinking to add the following indices:

fk1, fk2, fk3, fk_a1, fk_a2
fk1, fk2, fk3, fk_b1
fk1, fk2, fk3, fk_c1


since I'll be searching and joining to LegacyA, LegacyB, LegacyC independently and separately, these indices should be used correct?  Does this look like a sound approach?
You could possibly just use the first index and "include" the fk_b1 and fk_c1 columns. It will depend on your data so you'll need to test, as mentioned previously.