Is an investors personal dividend yield calculated based on the current share price or the buy in price?

Dividend yield is always calculated on the current (or expected if looking at future dividends) share price at the time of the dividend. You investment is worth $3 now regardless of what you paid for it, so the yield on your investment's value is 33%.

That said, remember that the stock price also goes down by the dividend amount, so a dividend isn't really "income" to you - your investment goes down by the same amount. If it makes you feel better to have a dividend yield of 50%, then your investment value also went down by 50% instead of only 33%. So there's no metric like "ROI" that applies to a dividend because it's really just a transfer of wealth from stock to cash.