Actually Webflow offers you a better way to style all H3s.
I don’t know if by that time when this post was created this feature was already available, but I will just write here anyway for future knowledge.
Solution:
When you select a H3 element, go to Style Selector on the left side properties and click on that textbox. There you will see a list of available classes and on the top in pink, you can see a base selector called “All H3 Headings”. Select that one and then style your heading as you want. Every styling you do there will apply to all H3 in your website.
