First thing - the compound found in poison ivy, oak and sumac plants that we are allergic to is urushiol oil. This is an oil and so does not dilute with just water. It takes another agent to assist in removing urushiol oil from clothes, ourselves, etc. One of the best agents is soap.
3 hours exposure is too long to have the oil in contact with your skin. For many people even just 5 minutes exposure is enough to set off the immune system response. I'm one of those who reacts when exposure is only a couple minutes. For some like my husband, he can wash off a half hour later and be ok (anything longer and he'll break out). You definitely want to wash off even if the exposure time is long as you don't want to continue to infect other parts of the body. Washing well with soap/water helps to reduce the reaction/rash.
Yes urushiol oil can get through clothes to the skin. That's why its recommended that plants be handled with pvc coated or similar gloves that will prevent the oil from seeping through the material. Make sure you wash all items that came in contact with poison oak so that you don't reinfect yourself.
Hope this helps!