Hi mummy, you may first want to try talking to her about why is thumb sucking bad. Since she is 3.5yo, she's at the age whereby she might understand to a large extend and would try to make a conscious effort to stop. For example, show her the videos/pictures of germs in their nails/finger. Tell her how she is no longer a baby, or explain how she might not be able to speak properly etc... Motivate and encourage her whenever she attempts to not suck her finger...like a reward system - stickers. Give her a soft toy to cuddle to sleep to keep her hands/fingers busy. Lastly, final attempt, you may try plastic finger guard (i.e leading brands like tguard).