The fact that tattoos can symbolise anything is part of their appeal. If you want a tattoo to represent the past, present, future or all three, first consider whether it is your personal experiences and aspirations you are depicting or a broader sociological view. If these ideas are not for you, hopefully they will at least get you started on the right path. Along with design, also consider the tattoo's proper placement and its colour scheme.
A personal past tattoo can be anything from a portrait of your mother when she was young to a native flag or symbol of your ancestry. Tattoos with broader sociological meaning can be anything depicting relevant and important happenings in the past, such as the Wright Brother's "Flyer" or a Tesla coil, which helped revolutionise electricity. The placement for the tattoo is entirely up to you, but consider using traditional shades of black and grey for the colour scheme to give it an antique feel.
Tattoos last forever, but unfortunately the present does not. A design that may be up-to-date today will be a "past" tattoo tomorrow. For this reason, present tattoos should instead be symbols that represent aspects of your life that will never change, such as your last name, your fingerprint, your birth date or your the name of your child. Tattoos with a more universal meaning for the present are far more difficult since it seems that nothing ever stays the same, but you can consider some abstract designs of constancy, such as the importance of money or love to you.
Tattoos that symbolise the future in a personal way for you should denote certain aspirations that will remain important to you even after you've accomplished them -- turning a future tattoo into a present one. These tattoos can also act as motivation for following through, such as the "scales of justice" if you are a law student hoping to become a lawyer. Tattoos with a more general message about the future allow for a level of artistic license and creativity and are almost always computerised or robotic in nature. Consider a circuit board cross-section of skin, implying you are a futuristic cyborg underneath or tattoo a bar code or binary code as though predicting we will one day all be categorised.
If you would like a tattoo that combines the past, present and future motifs, consider a particular design that allows you to do so. Choose a symbol of the past, such as the Wright Brother's "Flyer" towing a banner of your last name (symbol for a constant present) and armed with futuristic laser guns or being towed by a U.F.O. You can be as creative and outlandish as you want, in fact, the more off-the-wall your tattoo design, the better your chances of it being unique and original.