Three West Australians are being treated in hospital after being diagnosed with meningococcal serogroup W disease.

The trio of adults, who the Department of Health says are not linked, are the first reported WA cases in 2019 of the potentially deadly disease.

Meningococcal disease struck down 41 people in the state during 2018, including 30 serogroup W, eight serogroup B and three serogroup Y infections.