It would be something like this:

SELECT id_agendamento, hora, nome_medico, nome_paciente FROM tb_agenda AS agenda 
INNER JOIN tb_agendamento AS agendamento ON agenda.id_agenda = agendamento.id_agenda 
INNER JOIN tb_paciente AS paciente ON paciente.id_paciente = agendamento.id_paciente 
INNER JOIN tb_medico AS medico ON medico.id_medico = agenda.id_medico 

See more about making a relationship here

There are still some additions to the structure of your tables. It depends on which business rule you want to implement, but in most cases it would look better:

tb_agenda

: int

date : timestamp

time : time

tb_patient

pop_id : int

: string

tb_medico

Medical_id : int

medical_name : string

tb_agendamento