Gillette Co. is facing a lower-cost rival to its top-selling Mach3 razor from American Safety Razor Co. of Verona, Va. American Razor has started shipping its triple-bladed Tri-Flex razor to retailers in the U.S., Britain and Germany, a Gillette spokeswoman said. An American Safety Razor official declined to comment. Boston-based Gillette, which spent about $1 billion to develop and market the premium-priced Mach3, said it will obtain Tri-Flex samples for testing before considering legal and other options. The product lacks Gillette's marketing muscle, noted Carol Wilke, an analyst at Prudential Securities Inc. Gillette said Mach3 blades had a 16.4% share of U.S. sales in April, boosting Gillette's overall share to 70.3% of sales, up 3.7 percentage points from a year ago. Gillette stock fell 31 cents to close at $51.13 on the NYSE; American Safety Razor fell 6 cents to close at $14.06 on Nasdaq.