The synthesis of the Toffoli gate, Fredkin gate, three-qubit Inversion-on-equality gate and D(α) gate, as well as their implementation in a three spins system coupled with Ising interaction are investigated. The sequences of the control pulse and the drift process to implement these gates are given. It is revealed that the implementation of some three-qubit gates in a circular spin chain is much better than in a linear spin chain, and every two measurements of the quantum computation complexity are not always consistent. It is significant to directly study the implementation of the multi-qubit gates and even more complicated components of quantum information processing without resorting to their synthesis. three-qubit gate, three spins system, Ising interaction, the sequences of control pulses and drift processes PACS: 03.67.Lx, 03.65.-Aa, 03.65.FdThe realization of quantum information requires the accurate control of quantum states [1,2]. The active control of quantum states is realized by decomposing the evolution unitary matrix of the system into products of several realizable matrices. That is equivalent to exerting a certain control field on the system, and the goal of control is usually achieved by a sequence of control pulses [3][4][5][6]. The quantum information processes, especially the quantum computation, are usually described by the quantum circuit model [1]. Quantum logic gates are basic elements of quantum circuits. In 1995, Barenco et al. showed the universality of the set of one-qubit gates and CNOT gates [7]. The process of constructing quantum circuits by these elementary gates is called synthesis by some authors. The complexity of the quantum information processing can be measured by the number of elementary gates to constitute the quantum circuits, and it can also be further measured by the number of the control pulses and drift processes.The decomposition of matrix plays a very important role in the implementation and optimization of quantum gates.The methods of decomposition currently used in this field mainly are Cartan decomposition [8] based on group theory, cosine-sine decomposition (CSD) [9] based on numerical linear algebra and quantum Shannon decomposition (QSD) [10] proposed by Shende, Bullock and Markov. The most widely used forms of Cartan decomposition in quantum information processing are Khaneja-Glaser decomposition [5], concurrence canonical decomposition [11,12], odd-even decomposition [13], and a kind of Cartan decomposition for bipartite quantum system in high dimension [14][15][16]. Based on Cartan decomposition, the problems of the synthesis, optimization and "small circuit" structure of two-qubit gate are completely solved [17][18][19][20]. Much progress is made in the exploration of the optimal elementary gate number required to synthesize a general n-qubit gate [10,19,21,22].The best result is that the number needs at most 23 4 48 n 3 4 2 2 3 n − + CNOT gates asymptotically, which is obtained by the QSD [10]. But for specific multi-qubit gates, our work reveals the opti...