As one who learned how to do tography mainly using prime lenses, I'd say ditch the 3 you've got, and replace them with quality primes. You can pick up some great bargains second hand.
I like:
24mm (Wide angle for landscapes, architecture and that)
50mm (Indispensable fast bright non-perspective-distorting standard)
135mm (Portraiture; less obtrusive than an 85mm, better at isolating subject from background using shallower dof)
200mm (Longest that can be carried about without doing yer back in)
For full-frame 35mm format. Other people like different combos; a mate swore by a 35mm and an 85mm.
I find zooms, particularly cheaper ones, too bloody dark to look through in all but bright conditions, and almost impossible to work with in dim conditions. Mind you I am talking about using manual focus, so maybe not as important with hocus-pocus.
The 85mm 1.8 is a good recommendation; equivalent to a 135mm on 35mm.
Another option is something like a 70-200 2.8. Bit bulky, but loads better than those compromised f4-5.6 cheaper zooms. Should be a fair range s/h, and Sigma etc do decent versions.
With lenses, I'd always go for as bright as possible, and the best glass available. Cheap zooms are just not up to the standards of primes or fast 'pro' zooms. The worst are things like yer 18-800mm 'megazooms'. Significantly lower quality than using the 'right' lens.
Use primes. Learn to frame the subject better. Move around to get the best shot more. Zooms make people lazy, imo.
Photography is an expensive business....