The net charge of the system is -q.
The direction of the electric field at the positive charge is towards the negative charges.
The electric potential energy decreases as the positive charge moves towards the negative charges because the charges are of opposite sign and therefore attract each other.
If the distance between the charges is doubled, the electric force between them decreases by a factor of 4. This is because the force is inversely proportional to the square of the distance between the charges.
Yes, the positive charge can be placed in between the two negative charges. However, the system will be unstable and the positive charge will experience a repulsive force from both negative charges.