Police are investigating three separate armed robberies in Perth and Mandurah over the past three days.

The three robberies took place in Beckenham, Barragup and Upper Swan and are not believed to be linked.

A man believed to have a gun demanded money from a staff member working at a restaurant on Albany Highway in Beckenham.

He fled on foot with the cash register till tray at about 9pm on Friday night and was last seen running across Albany Highway towards a car park near the Kenwick Link.

Police said the staff member was not injured in the robbery.

The man was wearing a dark jumper and is described as being fair skinned, about 35-40 years old and about 183cm tall with a medium build.

A second robbery happened at a service station on Pinjarra Road in Barragup, near Mandurah, at 12:45am on Sunday.

A man threatened a staff member with a stick before he fled on foot with some cash.

The staff member was not injured.

A third robbery happened just before 3am on Sunday at a roadhouse on Great Northern Highway in Upper Swan.

A man threatened staff with a screwdriver and demanded money before he left with some cash.

He is described as being about 45-55 years old, fair skinned and about 170-175cm tall.

He was wearing dark clothes during the robbery.

Anyone with information which could assist police should call Crime Stoppers on 1800 333 000 FREE.

Extracted in full from news.com.au.