I Agree with Dave.
Option 1
- better in some circumstances (if it makes make existing investors happy that they are carrying less risk, or have made a small profit ).
- If the opposite applies, then it's worse!
- Also, if the big investor ends up being a 'whale', this may be unwanted.
The price is probably best left to negotiation between buyer and seller, unless you have placed some rules. Beware of rules trying to control people too much - they can make informal agreements to get around them (grey markets).
Option 2
Could be the way to go if you don't want whales, but otherwise just deprives you of cash. Dave's idea could work, but be wary of ending up with large amounts on such arrangements - financial regulators don't seem to like that.
Option 3:
Always possible. No guarantee, of course, that the extra money will be available when you are ready for it.