Can I set a merchandise plan with three options and three different prices for instance ? but which would set three different figures in the same CB field ?
No, you'd need 3 different merchandise plans.
You could try using a Donation plan then set "Are donation amounts given ?" to "Yes, only a list of suggested amounts" then specify your 3 prices in "Suggested amounts (separated by comma , )". I'm not sure how you'll increment the field based off the donation price they selected though, but you could try an IF substitution against PLAN_PRICE or PLAN_RATE, but I'm not sure if that'll work right for donation plans.