Great Western will buy an Arizona thrift.

Great Western Financial, Beverly Hills, the nation’s third-largest thrift holding company, said it has agreed to acquire a small Arizona saving and loan that it hopes to build into a major presence in that state. It will purchase Phoenix-based First Commercial Savings & Loan Assn. for Great Western common stock worth about $17 million. First Commercial, with $76 million in assets, has only one office in Phoenix, plus a mortgage-lending outlet in Tucson. Great Western, with assets of $27 billion, has more than 800 offices nationwide.