No. They are conceived on the same date.
try ultrasound.
When you do the ultrasound they see it.
An Ultrasound can be done to find out.
At the first ultrasound
The only sure way of knowing you are pregnant with twins is by ultrasound. An ultrasound at 8 weeks would show if you were carrying a multiple pregnancy.
Ultrasound once you're pregnant.
You are having twins
Three weeks pregnant is a week before you miss a period so it is impossible to know it is twins. If you are three weeks after conception, (5 weeks pregnant)it is still early to see a heartbeat.
You will find two heartbeats at the ultrasound.
You will probably find out when the doctor runs the ultrasound and two bodies pop up.
Having sex is a good way to start to conceive. However, conception of twins depends on the splitting of the egg, or upon several eggs being produced at once. Twins often run in families, leading to the strong possibility that the conception of twins is a genetic feature more than happenstance.
The ultrasound is as conclusive as it gets. The tests have a small margin of error because they test for hormones. The ultrasound sees what is there or not. You are most certainly pregnant. Possibly with twins. If you are still early on the ultrasound can be inconclusive about that. Sometimes one of the twins will be reabsorbed into your body early on as well. I know one person that showed twins early on but only one in a later ultrasound. I hope that this is good news for you and that both babies are born healthy.