x$ = "/home/user1/tmp/coverage/test"
y$ = "/home/user1/tmp/covert/operator"
z$ = "/home/user1/tmp/coven/members"

a = len(x$)
if a > len(y$)  a = len(y$)
if a > len(z$)  a = len(z$)
for i = 1 to a
    if mid$(x$, i, 1) <> mid$(y$, i, 1)  break
next i
a = i - 1

for i = 1 to a
    if mid$(x$, i, 1) <> mid$(z$, i, 1)  break
next i
a = i - 1

if mid$(x$, i, 1) <> "/" then
    for i = a to 1 step -1
        if "/" = mid$(x$, i, 1)  break
    next i
fi

REM Task description says no trailing slash, so...
a = i - 1
print "Common path is '", left$(x$, a), "'"