By Costas Pitas and Lefteris Papadimas ATHENS (Reuters) - Greece will reopen its banks on Monday for the first time in three weeks, but with restrictions on withdrawals still in place, German Chancellor Angela Merkel said the government would have to move fast on any new bailout terms. Greeks will be able to withdraw 420 euros a week at once instead of just 60 euros a day but capital controls will remain. Greek Prime Minister Alexis Tsipras is trying to turn a corner after he reluctantly agreed to negotiate a third bailout, allowing the European Central Bank to top up emergency credit lines but prompting a rebellion in his leftist Syriza party.