Reddit reviews Shimano Tourney ST-A070 3 x 7-Speed STI Lever Set

Here's how you'd price it out at a bare minimum, it won't be "cheap" and ultimately if you end up getting more invested in the sport then you'll want to move on to a dedicated road bike anyway. Here are some of the lowest priced options to do this.

Shimano Tourney Shifters $80

Drop Handlebars $25

Shift Cable Set $9

Bar Tape $7

Problem Solvers Travel Agents $18

Stem $15

Total: $154 + Shipping,
Additionally a set of allen keys if you don't have those.

They would likely be a problem. The thing with most modern handlebars is the tube diameter gets larger closer to the stem so you wouldn't be able to mount them up top and if you could they likely wouldn't clear the space in between the drop section. You could mount them on the bottom of the drop bars but this would be awkward and could easily result in a crash. You are best off getting drop bar specific brake/shifter combo or "brifters". Since it is a 7x3 speed bike you will be limited for shifters and will have to get the Shimano Tourney brake/shifter set like here or a MicroShift set.

3x7? Tourney or RSX

link to what was looking at https://www.amazon.ca/Shimano-Tourney-7-Speed-Triple-Lever/dp/B00GQCC9A0/ref=sr_1_fkmrnull_1?keywords=shimano+7+speed+sti&qid=1556179209&s=gateway&sr=8-1-fkmrnull