The robbery took place at the store on Oldbury Green Retail Park just after 7pm on Friday.
No one was injured during the robbery, police said.
A spokeswoman for West Midlands Police said: "We are investigating an armed robbery at the Poundland store in the Oldbury Green Retail Park just after 7pm on Friday.
"Three men armed with knives entered the store and forced staff into an upstairs area.
"A safe was then opened and the men made off with a quantity of cash.
"Fortunately, there were no injuries caused to any staff members or members of the public.
"We understand this was a very distressing incident for those who witnessed it and we are urging anyone with information to contact us via live chat at west-midlands.police.uk between 8am and midnight, or call 101 anytime.
"Crimestoppers can be called anonymously on 0800 555 111. Please quote crime reference 20SW/286757E/20 or 20SW/286751K/20."