you can use line-height property

    <table width="600" cellspacing="0" cellpadding="0" align="center">
  <tr bgcolor="#f9c9d5">
    <td></td>
    <td rowspan=2>
      <img src="bimage/box.jpg" alt="box">
    </td>
  </tr>
  <tr>
    <td colspan=2>
      <h3 align="center" style="font-size:20px, line-height:0"> WELCOME</h3>
      <p align="center">Jinho</p>
    </td>
  </tr>
</table>

It is not recommended to use tables and inline styles, but perhaps you meant this?

<table style="width:600px; height:132px; border-spacing: 0; border-collapse: separate; 
background-image: url(https://i.stack.imgur.com/B49ut.jpg)">
  <tr>
    <td style="text-align:center;">
      <h3 style="font-size:20px; margin-bottom:0">WELCOME</h3>
      <p style="margin-top:0">Jinho</p>
    </td>
  </tr>
</table>

I’m doing a text over an image and learning how to do the text size and position so it will look align, and right now I really don’t know how to make the gap in between the h3 and p smaller.

I don’t want to use CSS if possible I want it to be in HTML only just using the style in the code.

If you have a better way to do text over an image and multiple sentences with adjustable gap size and use each sentence use different font. It will be great.

Image from desktop

<table width="600" cellspacing="0" cellpadding="0" align="center">
  <tr bgcolor="#f9c9d5">
    <td></td>
    <td rowspan=2>
      <img src="bimage/box.jpg" alt="box">
    </td>
  </tr>
  <tr>
    <td colspan=2>
      <h3 align="center" style="font-size:20px"> WELCOME</h3>
      <p align="center">Jinho</p>
    </td>
  </tr>
</table>