There are only two standard choices in MATLAB for floating point computations, and two ways to get round the problem.
That is, you can use single or double for floaing point numbers. But even there, you need to be careful, as MATLAB defaults to a double. You cannot set some flag in the top line of your code that says "From here on, ALL new numbers created, and all precision in your computations will be done in K number of digits". You cannot even set single precision as the default.
Having said that, there are two ways out. You can use the symbolic toolbox.
X = vpa(pi)
X^2 - 3
Or, you can use my HPF toolbox, available for free download from the file exchange.
DefaultNumberOfDigits 6 0
X = hpf('pi')
X^2 - 3