i have got 56% in boards and not expecting good marks in cet which engineering i should apply any low rated college suggestion????

First of all it's completely okay don't worry about college rather select the course you like. Then select colleges that have that course make sure you select 3 courses you'd like to complete and accordingly choose 12 colleges that host these courses. Now talking about the colleges you'd have a good chance to get into as per 2018 trends anyone around 80+ could get CMPN in rizvi . If you're looking for good colleges with streams less in demand like extc or electrical then with 90+ you could get St. Francis. Incase you're having some reservation religious then you can easily get into certain colleges.
Universal, rizvi,vartak,shah and achor, are always available for low grades remember these names
See since you have said you didn't score well in cet nor your boards here a decision you will have to make
Take A grade subjects like Computer or it in a low grade college
Or take B grade subjects like electronics and electrical that have lesser scope of selection then cmpn and it but here you can get a better college in the top 20s -30s if your cet score is above 90.
The names of few colleges I've said there you can choose any subject but they aren't popular not are they in top20s.
While putting up your preference list you have to decide does college matter more or course
If course matters more select course then add colleges according but be ready to get the course is colleges like rizvi or saboo siddique or universal etc etc
If college matters then you should add college and it's course serially here you could land in Francis but in electrical or extc stream .