This is a probability reference. A 2:1 spot would be a situation where event "p" occurs twice as much as event "q". Paying 3:1 refers to a gambling or investment wager that pays out three dollars to every winner who bet one dollar. Finding a 2:1 event that yields the investor a 3:1 payout would give the investor a highly profitable long-term arrangement. Thus, an investor who holds his money until he finds these advantageous situations would be good at "picking the right spot".

This is gambling terminology. The use of the word "shot" here is a specialized usage, meaning a bet or a chance or an attempt. Note also the word "longshot" uses "shot" in a similar sense. To "pick your spots" means to be selective, and choose to gamble only when the odds are in your favor, that is, when the probability of success and the payoff from succeeding outweigh the risks.

"To find a 2 to 1 shot that pays 3 to 1" This is the language of gambling. A "2 to 1 shot" is an investment that returns double what you bought it for. Similarly, a "3 to 1 shot" would be an investment that returns triple what you bought it for. From the context, it would appear that they are talking about something that is inaccurately rated as returning double your investment, when in fact it will really return triple your investment. "Pick your spots" This is similar to "cherry-picking" or picking only the best of something. Buffett (presumably Warren Buffett?) is saying that you cannot analyze every single "horse" (meaning investment opportunity). Instead, you have to be able to have some criteria to identify what the best investment opportunities are of all the ones that are available. Please let me know if these explanations help you.