It is nearly impossible to think that 3 different lenses across manufacturers all have the same kind of defect in the same area of the lenses. Dust I could believe, but not optical imperfections/chips. Also, anything that small in the middle of your lens WON'T IMPACT your photos. Want to try a test? Cut out a 2mm x 2mm square from a postit note, and stick it in the same area as that "speck". I suspect after you take some shots with it, with that piece of yellow paper on your lens, you won't see it in your photos.
Finally, having a 55-250, I can tell you that if you look through the inside of the lens at just the right angle, you will concentrate a lens flare such that it bounces off the edge of the metal edges that hold the lens elements in place, making it look like a bright metal flake or chip. If you hold the lens up and look through it straight on and see nothing in the pathway, there is no issue. Angling the lenses to see what you can see on the outer edges of the inner groups of elements isn't going to help identify issues.