You can use outerjoin to do that. Your keys would be the three common variables and since you only want to add the extra identifier variable from a to b, you can specify MergeKeys to be true.
a = readtable('a.xlsx');
b = readtable('b.xlsx');
outerjoin(a,b,'Keys',{'height','weight','width'},'MergeKeys',true)
ans =
4×7 table
weight height width identifier serialNo age cost
______ ______ _____ __________ ________ ___ ____
502 101 52 1 1 34 4322
502 101 53 NaN 4 55 5533
515 101 53 3 3 43 2344
515 131 53 2 2 23 5433
