3 weeks post from a lift and the stitch seems to be splitting at the breast fold. Should I be worried it will scar more? (Photo)

It looks like you have dehiscence of the incision.  The tissue underneath looks healthy and I don't see signs of infection.  This type of wound should heal on it's own, but it will leave a larger scar.  I would be concerned, however, if you have an implant as it could be at risk depending on the quality of your tissues.  

You have some delayed healing and separation of your wound edges at the bottom.  If the separation is just superficial and you have healthy tissue underneath, these areas are capable of healing on their own with wound care over time.  It would be best to see your surgeon for an assessment of the area to find out if this is your case.  If the separation is not just superficial and extends deep to your implant pocket, then there is a threat of an implant infection.
In terms of scarring, areas that have separated tend to scar more than areas that have not, but fortunately it has occurred at the bottom of the breast, an area that is often not noticeable once you have fully healed.  In addition, it appears as though you have a vertical/lollipop scar, in which the excess vertical skin of your breast has been gathered, rather than directly excised.  If you are unhappy with the healing or scar at the bottom of your breast, you can always convert to a small anchor scar, which can remove the scar and excess skin directly, improving your breast shape at the same time.
I hope this helps.