Editing size of h3 tag in wordpress on a one-off basis

If you’d like to use <h3> tags that are the same size as your <h4> tags only on one page, add a specific class to each of the <h3> tags you’d like that size.

In the Classic Editor, you should have a “Text” tab where you can input custom HTML.
Location of "Text" tab
In there, type your <h3> tags with a custom class:

<h3 class="small-h3">Your text here</h3>

Then go to the “Appearance” > “Customize” tab in the WordPress dashboard. On the screen that shows up you should have a “Additional CSS” tab:
Location of Additional CSS tab

Type there your custom CSS for this style:

.small-h3 {
    font-size: 1.25em; /* or whatever size your h4 tags are */