Not exactly a Queen but the regnant duchess
Anne of Brittany.
She married by proxy Emperor Maximilian of Austria and successively two french kings Charles VIII and Louis XII.
The championess should have been Queen
Johanna I of Naples (1326-82).
She married 1st Andrew of Hungary (m.1333-died 1345), 2nd Louis, Prince of Taranto (m.1347-died 1362), 3rd James IV, King of Majorca (m.1363-died 1375) and finally 4th Otto, Duke of Brunswick-Grubenhagen (m. 1376, who survived her).
Who were the queens regnant in European history who married two or more times? Off the top of my head, I can name only two -- Mary, Queen of Scots (who had a total of three husbands in her life) and Maria II of Portugal. Neither had any children from her first marriage; in each case, it was the second marriage which produced heirs to the throne.
Can anybody name other examples?