Access your Rooms section and click on Change Rates as you see here:
Then double click the main rate (in yellow) to create the new one.
A new popup will appear, then click on New Rate:
In the new window insert the following information:
-Edit the name of the new rate, for example "3pax"
-Choose the number of guest, for example choose 3
-Flag the characteristics the new rate should have (not refundable? breakfast included?)
-Create a price rule: if you want to decrease the price insert -10 (for example); if you to increase the price insert +10 (for example)
Choose this symbol if you want to insert a difference in euros (or whatever is your currency):
Choose this symbol if you want to insert a difference in percentage:
When you have completed all the information click on Save
Easy, isn't it?