I want to go from the bus station to Paseo de los Tristes by bus because I am staying at the Cervantes apartment.
On the bus pages on https://granadainfo.com/buses.htm
it says that I have to use 3 buses.

It says that the price per ticket is 80 cents per person with a bono but 1.30 with a normal ticket.

Do I have to buy 3 tickets is can I do the journey with 1 ticket?
Can I buy a bono at the bus station or on the bus?

Nowadays you only need 2 buses.

When you get on the first bus the ticket is timestamped.
You can then get on any other bus as long as it is not on the same line for up to 1 hour afterwards.

It is called a trasbordo. 
You only have to pay once.

You can buy a ticket and a bono on any bus in Granada


Get the 33 to the cathedral
Then get on a C31 or C32 to Paseo de los Tristes.

There is information about the C buses  here 

https://granadainfo.com/alhambra/minibus.htm