In general losing weight is the basic burn more calories than you consume each day metric. You need to burn about an additional 3500 calories to lose 1lb of body weight. If your Soylent consumption is about 500 calories under what you burn each day you will lose a steady 1 lb each week. 3lbs in 3 days is not likely actual “lost weight” (obviously you are 3lbs lighter but I doubt you were able to burn an additional 10500 calories…) however it is the change in the amount of water being retained by your body.
I weigh myself each morning (been trying to cut the weight a bit myself and have done well with DIY) and have seen days where I have suddenly shot up 3lbs (and I did see some sudden drops when I first started doing DIY). I have found a direct correlation to these sudden weight swings (plus or minus) with my intake of salt. I usually eat my diy soylent for breakfast/lunch and then have normal dinners with the family during the week, weekends are a bit less regular. If I have a meal with considerably more salt then I normally take in I will see a sudden increase of a couple pounds the next moring when I weigh myself (and I know I did not consume 7000 extra calories in a single day!). Basically the increase in salt consumption made my body hold onto more water. I’ve seen statements saying that the first 10lbs or so of weight loss can just be “water weight” and this is why when you first start a diet you can sometimes see very quick results in the first week or so but things slow down and the weight loss gets more difficult. (not sure if this is 100% accurate but it sounds likely)
I would suspect that the quick 3lb loss that you saw is a result of the Soylent you are now eating having a much lower salt content then the normal meals you had previously been used to so your body is holding on to less water.
While Soylent is not a “weight loss” product it is a great tool for losing weight just due to the ability to exactly dial in and know precisely what you are consuming each day. When I first started using DIY and looking to lose some weight I got a fitbit and used it to figure out what I was burning for calories each day (when you have a desk job it is depressing to see how little activity you actually do each day). After averaging the numbers out from a month of data I adjusted my diy recipe so that I would be about 500 calories below my burn rate. It is slow and steady but I’ve averaged about 1 lb of weight loss per week (sometimes more if I’m more active) it doesn’t seem like much at first but after a few months the results can be impressive.