So I'm not a step-parent but with multiple kids, it's also a big issue. Sometimes you feel like you are always doing things as a group and have completely lost the relationship you had before. I have three tricks.
1) I schedule dates. I go on at least one date with a kid every month, and they are often multiple hours (so we can do more than one thing) and they are always what the KID picks. Last month one kid wanted sushi and shopping for needed school clothes, another wanted ice cream and a movie, and another kid (the youngest) wanted to go to the park (we brought a packed "lunch" there -- something that signifies being a "big kid").
2) I stagger my day. The oldest wakes up first and goes to bed last so we eat breakfast together and read books together, even though the school day is the longest. The youngest gets me during the day when the two oldest are at school, and I postpone that nap until the middle one gets home from preschool so I can get time every day with all three (although not the quality time as our dates).
3) Finally, I make sure that I always bring one kid but not all of them on the weekends for errands. The others feel left out but I remind them that only they get to go next time. This also isn't "quality" time in the same way but because I shop at an Amish market on Saturdays, it's not like going to Costco either. The other type of errands I save for weekend one-on-one time with my kids are: a) going to the library, b) going holiday shopping for family members, c) doing kid-specific things like picking out the new glasses. That way they are "special" and every kid loves that time together and I still get the errands done that needs to be done (in a less stressful way for me!)>