I know what you mean. I have 2 girls and have taken them to Disneyland four times in the past 5 years and every time, I went there, I had to ask someone to come with me because I didnt want to leave one of my girls alone in a ride.
I will take them to Disneyland and Universal Studios next month and I dont have anyone to go with us
I will probably look for rides that allow in 3somes, like the teacup ride which is my FAVE!!! No matter how old I get, I still will always love teacup rides
But it does get easier once they get older and more independent so you dont have to worry about it so much.