its 13 people by Thursday
Explanation:
if Ethan send the email to 3 people on Monday that means he also had the email on Monday so that's 3+1=4 and the next day would be Tuesday that's 3 more people and 3+4=7 so by tuesday there would be 7 people then theres Wednesday that's 3 more people if you do 3+7=10 that's 10 by Wednesday then theres thursday 3 more people 10+3=13 that's how I got my answer to you