You cannot use a 5.1V Zener diode in a regulator for charging a 3V battery...
And Zener regulators are pretty sloppy anyway -- in general it's a lot better to just use a 3-terminal linear regulator. And battery charging can be a fairly complicated matter, depending on which battery chemistry you are working with...
Since you didn't give many details, here is the scenario:
If you really insist on this approach, do it out doors, nothing around that you want to keep (like people, animals, or houses), and don't mind destroying a battery, then a 35Ω, 50Watt resistor would be about right.
Oh, and stay up-wind of it.
I really recommend that you take heed of the approach by @berkeman though.
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