$5K is way too much for a an average cow. Its very cheap for a good one - especially a three way. There are no guarantees you'll ever make your money back on paying that much for a cow. My only thought is that if she was really good, why is he wanting to sell her? Might be a valid reason, but I'd ask.
Another question to ask yourself, - if the cow drops dead tomorrow, will it cripple me financially? Don't put too many eggs in one basket. If you can afford to lose it, it might be worth the chance if she's good.
We sell a lot of three way heifer pairs for $2000-$2500 every year. Nobody would touch my best ones (top 5% of the herd) for that though, but we have a very long term view on those kinds of things though and we are understocked.