S Corp in Oh -Want ot sell more shares to a shareholder

We are S corp in Ohio with 3 shareholders. Shareholder A has 100 shares, B has 100 shares , and C has 50 shares. Our intent 3 years ago was to make all three of us equal owners but we failed to do the proper procedures to do this. We have just recently been made aware of this - and now one of the shareholders does not want to sell 50 shares to shareholder "C" to make him equal. (The Corp has 600 shares )
Question - To sell 50 shares of the corp stock do we have to have all shareholders agree or could the 2 shareholders "B" (100 shares) & "C" (50 shares) be able to authorize the sale of 50 shares to shareholder "C". Reason this is our desire is to make right our promise of 3 years ago...In fact we had advised are accountant that eff Jan 06 we were three equal partners and the last three years K-1s have reflected this. - Is this your question? Add additional information