Section 3. A trustee may resign by an instrument in writing delivered to the transferor or to his guardian or conservator, if any. A trustee may be removed by the transferor by an instrument in writing delivered to such trustee. If there is more than one person serving as trustee, a vacancy need not be filled, and until a successor is appointed the remaining trustee or trustees may act alone. In the event of a vacancy a successor may be appointed by the transferor, if legally competent, or as the transferor shall have provided by a written instrument, and otherwise by his guardian or conservator, if any, and if none, by his heirs presumptive, and such appointment shall become effective upon acceptance.