I haven't used caster oil, but if I recall correctly, isn't it a laxative? If so, I would probably be careful with that.
I kind of doubt that you want to have diarrhea right now (wince). My suggestion for whatever it's worth? Unless you are having specific problems that make it important to deliver asap, you may just want to wait this out and rest as much as possible right now. With my third son I was so uncomfortable for so long before his delivery that I was really worn out when it came time to labor. We both still did fine, but I sure could have used some more energy during labor. So my best suggestion is to put your feet up or take a nap (with or without hubby).
Since you're 3 cm & having contractions 3-4 minutes apart (even off & on), I'm thinking that you'll have that baby in your arms soon anyway & it would be nice to have had some rest first.
We're crossing all fingers & paws for you here for a fast & easy labor.