Dear no problem of 3 cylinder with CNG, as all MEHRAN's going well on CNG.
i also owned Cultus 2004, 3 cyl 993 CC.
on of the most common reason of LOW PICK of cultus is Hydraulic tappets.
normally replacement period is about 60,000 to 80,000 KM.
after that pick will inshallah restore.
check it from reliable mechanic.