How to Add Emoji to Conditional Formating Icon?
1. Get emoji
1) Get emoji by "Win+."

Then write a measure:
Emoji = IF([Rank]<=3,"😁","😟")
2) Get emoji by UNICHAR:
UNICHAR = IF([RANK]<=3,UNICHAR(128513),UNICHAR(128577))
2. Combine values with emoji

Emoji couldn't be added to conditional formating icon directly, because an emoji is a text.
So you need to convert emoji into an image. How?
Wrap Emoji into SVG:
Emoji_SVG=
<text x='24' y='26' text-anchor='middle' dominant-baseline='middle' font-size='40' fill='Tomato'>"
&IF([RANK]<=3,"😁","😟")&"
</text>
</svg>"
UNICHAR_SVG =
<text x='24' y='26' text-anchor='middle' dominant-baseline='middle' font-size='40'>"
&IF([RANK]<=3,UNICHAR(128513),UNICHAR(128577))&"
</text>
</svg>"
Then, add the SVG emoji measure to conditional formating icon, you could get the result.