You have several choices, but choose one and stick with it, if you change the rules he will become confused and upset .
1. Persevere! Now you have managed this far you have to let him know that you wont give in. If you do give in, the message he is getting is that if i keep crying and refusing to drink, mummy will give in. You can get extra fluid into his diet by giving ice lollies, milk on cereal, gravy etc.
2.If you are really worried, then give in immediately, put away all the beakers and dont mention them again. Wait until his birthday and buy him a 'big boys' cup with his name on. Tell him that now he is three he isnt allowed to have a bottle anymore (Hide the bottles - or better still throw them away if you can). He will be that bit older and may accept this with no problem if you are really lucky!!