Q-switched fiber lasers emitting at around 3 μm have been widely applied in various scientific and industrial fields, such as molecular spectroscopy, laser surgery, material processing, remote sensing, and mid-infrared (mid-IR) supercontinuum source generation. Au nanocages (Au-NCs) have attracted much attention recently due to their outstanding saturable absorption properties including broadband absorption, ultrafast optical response (a few picoseconds), and large third-order optical nonlinearity coefficient that caused by local surface plasmon resonance (LSPR). We propose and demonstrate a tunable Er3+ doped ZBLAN fiber laser using Au-NCs as a Q-switcher for the first time. Nonlinear absorption of the Au-NCs was measured by a home-made Ho3+/Pr3+ co-doped mode-locked fiber laser at 2850 nm. The measured modulation depth, saturation intensity and non-saturation loss are 10.73%, 0.11 MW/cm2 and 3.26%, respectively. The central wavelength of the Q-switched pulses could be tuned across 54.1 nm (from 2753.0 to 2807.1 nm). The Q-switched fiber laser delivers a maximum average power of 253.7 mW with corresponding pulse energy of 4.06 μJ and pulse width of 1.30 μs at repetition rate of 62.5 kHz. Our work shows the Au-NCs are promising saturable absorbers (SAs) for 3 μm mid-infrared (mid-IR) pulse generation.