Walgreen Co. today made an offer to buy Longs Drug Stores Corp. for $3 billion, a price it said represents $3.50 per share premium over what CVS Caremark Corp. is offering for Longs.

The transaction would include all Longs' retail drug stores in California, Hawai'i, Nevada and Arizona, as well as its RxAmerica LLC subsidiary, Walgreen Co. said in a news release.

"We believe the combination of Walgreens and Longs is a highly compelling transaction that is superior to the pending transaction with CVS/Caremark, accelerates Walgreens' expansion into high growth markets and delivers meaningful cost synergies," said Jeffrey Rein, Walgreens' chairman and chief executive officer.

Walgreens last year opened the first store in Hawai'i and said it had plans to open 30 to 40 more across the state.

Walgreens said it was offering $75 a share for Longs' stock, compared to the CVS offer announced last month of $71.50 a share.

Walgreens made its offer after the biggest shareholder of Longs Drug Stores Corp. said it won't tender shares to CVS Caremark Corp. because it didn't think CVS' $71.50-a-share offer was enough.

The comments from Advisory Research Inc., which holds about 9.2 percent of Longs shares, come one day after another investor, Pershing Square Capital Management LP, said it won't back the CVS purchase and called for an "open, competitive" sale.

CVS Chief Executive Officer Tom Ryan told CNBC that the company is "not moving" on its bid price.

The Walgreens' announcement was made after the stock market closed. Shares of Longs Drug Stores soared to $74.50 in after hours trading after finishing the regular session down 7 cents at $71.66.