Create a formula like this:
numbervar array x := [{table.field1},{table.field2},{table.field3}];
numbervar med:=0;
if
(
(
x[1]>=x[2] and
x[1]<=x[3]
) or
(
x[1]<=x[2] and
x[1]>=x[3]
)
) then
med := x[1] else
if
(
(
x[2]>=x[1] and
x[2]<=x[3]
) or
(
x[2]<=x[1] and
x[2]>=x[3]
)
) then
med := x[2] else
if
(
(
x[3]>=x[1] and
x[3]<=x[2]
) or
(
x[3]<=x[1] and
x[3]>=x[2]
)
) then
med := x[3];
med
-LB