170ml is a lot of milk for a little baby Is the one who doesn't drink well the smallest? Is she perhaps not getting tired (sucking is hard work). Is there any way she could have smaller bottles on a 2-hrly routine for a few weeks to help her a bit? My daughter was breastfed but on occasion had an expressed bottle. up until 5 months she was eating on avg every 2 hrs and about 100ml. At 3 months she was only taking 60ml/feed. I'm just telling you so you know that not all babies take huge bottles all the time

Usually a baby who is very tired doesn't take in enough milk, gets more tired, takes in less milk etc The solution is usually smaller feeds more often. Perhaps it could work for you?
As Claudia posted, babies instinctively know how much and when they need to eat. I know they were prem and it is a worry if your little one doesn't take her bottle. Is she thriving? Is she still growing well? (not weight so much, but does she look healthy, happy and alert like a 2 month old should? (working on corrected age here). If they are all growing well and are sleeping through the night and they all seem happy and healthy then I wouldn't worry about making sure they get extra calories from anywhere, they are clearly getting all they need from their milk. Also, don't forget that the amount of milk they need is different for each baby and the guidelines on the formula tin are just guidelines and should also not forget to work it out based on weight rather than age.
Before adding anything to bottles rather speak to their paed about your concerns and perhaps he can give you some guidance.
I don't want to sound horrid, I've worked with too many babies who are 'encouraged' to always finish their bottles / meals and nearly all of them have some issue surrounding food so this is a big concern of mine. But, having also looked after prem babies and multiples I do understand your worries.
Another option for the little baby is to introduce a dream-feed - this is where you offer a bottle at about 10pm without waking baby and let her drink till she stops. It might help you, but if she won't take it don't stress, not all babies do.