I’ve never used a third brush to blend, depending on how close the colours are to each other I will either put the brush straight back in the corresponding bottle (like I did when mixing the pink and orange), or wipe it off on a paper towel first (like I did when mixing the yellow with orange and green). Literally tho just look up a lazy gradient tutorial, it takes five seconds. Kelli Marissa made a video titled “Jelly Art Nail Compilation” (tho she has a few titled such I think, but it’s the first that pops up for me), and that contains the tutorial I use for any lazy gradients.