credit card
信 · 9 strokes
Conveys trusting (信) what a person (人) says (言). 人 is also a sound component.
用 · 5 strokes
Depicts a bucket with a handle on the right side. Based on the original meaning "bucket". The meaning of this character has shifted over time and now means "use".
卡 · 5 strokes
Origin unclear, but typically explained as being stuck between something above (上) and something below (下). The uses of 卡 to mean "card" or "calorie" are foreign phonetic loans.