paremeters in selection screen

Hi,

i have 3 parameters p1 p2 p3

p1 is OBLIGATORY and p2 p3 is not

select *

into wa

from link

where f1 = p1 and

f2 = p2 and

f3 = p3.

the sy-bubrc = 4.

but in table link ,there are values .

how to write this. i don't want to write this select laguage several times