On the chart in the wizard chapter, there is only spell slots, not spells known.
PHB p. 114, under the Preparing and Casting Spells heading. 2nd paragraph, 2nd sentence
...To do so, choose a number of wizard spells from your spellbook equal to your Intelligence modifier + your wizard level (minimum of one spell)...
If you're asking about how many spells they should have in their spell book, also on p.114, Under the heading Spellbook at the top of the page,
...You have a spellbook containing 6 first level wizard spells...
and at the bottom of the page under Learning Spells of 1st level and Higher,
Each time you gain a wizard level, you can add two wizard spells of your choice to your spellbook.
So, assuming you have a 16 Int, a third level Wizard can prepare 6 spells (3 int mod + 3 level), and your spellbook should contain 10 spells, probably 8 1st level and 2 2nd level.