Something

select 
  customers.customerid,
  sensors.sensorid,
  max(events.[timestamp])
from customers 
  left join sensors on sensors.customerid = customers.customerid 
  left join events on events.sensorid=sensors.sensorid 
where events.customerid=customers.customerid and events.sensorid=sensors.sensorid and events.eventsourceid=1 
group by 
  customers.customerid,
  sensors.sensorid