I don't know whether I have to put the question in the decision node or above on the control flow? In my case, the Agree to Price should it be on the decision node or on the control flow arrow that inputs the node?

Problems should be placed at decision nodes.Back to your example,“the Agree to Price” should be placed at decision nodes, that is,on the decision node，not on the control flow arrow that inputs the node。

As well as that, if a use case only specifies a condition, such as " If a vehicle is available, the customer is offered the vehicle and a price is quoted. If the customer accepts then a rental is initiated." how does the decision node look like?

According to the conditions you provided, I used ProcessOn software to draw the decision nodes you described for your reference.

Moreover, what if there are three different actors, should there be a swimlane representing the actors or can the activity diagram be drawn without one?

UML activity diagrams can clearly describe the business or processing process, but they cannot clearly express who completes these activities or actions. Swimlanes can help us solve this problem. They group activities or actions according to the objects they are executed, and separate each group with swimlanes. In this way, the transfer of activities or actions is clearly described, and it is also clear who completes these activities or actions. Therefore, whether there are two participants or three participants, swimlanes can be used for better presentation, which makes reading more intuitive and clear.