How can you ever buy a development card with 14 tokens? as you can never reach this amount, even when you draw +3 on your turn, the max you can reach is 13, and you cant use them to purchase the card as your tuen is over?
I don't think players are meant to by a level 3 development card with just gem tokens. As you righty say, buying a development card costing 14 or more with just gem tokens would be impossible. Are you are aware that development cards count as bonuses? If you look at the rules here they say-
The bonuses a player has from development cards acquired on previous turns provide discounts on the purchase of new cards. Each bonus of a given color is equal to a token of that color. Thus, if a player has 2 blue bonuses and wants to purchase a card which costs 2 blue tokens and 1 green token, the player must only spend 1 green token. If a player has enough development cards (and therefore bonuses), they can even purchase a card without spending any tokens.
So a player can buy a development card costing 14 tokens provided that the cost is reduced by bonuses for previously bought development cards.