I notice that people usually use two allele-specific oligonucleotide probes (ASO) on SNP arrays to distinguish two alleles for a certain SNP position. I wonder if there is a SNP array platform that can distinguish more than two alleles for a certain position (in dbSNP, there are 3 alleles for some SNP positions). Thanks!
Modern arrays, to my knowledge, have all four combinations per SNP on SNP chips.
But they're kind of going out of fashion these days in favour of Seq.
Arguably the best chip on the market, Affy's 6.0 chip: http://www.affymetrix.com/estore/catalog/131533/AFFY/Genome-Wide+Human+SNP+Array+6.0#1_1
Your best bet is to fire them an e-mail and ask :)