a cent is 100th of semitone, so there are 1200 cents in an octave.
The cent as a ratio to the semitone is something like ([sup]12[/sup]√2)/100, which is something like 0.01059 of the frequency.
Shift the pitch up by 3 cents you are going to do the following (assume Concert A = 440 Hz)
440 Hz + (440 Hz x 0.01059) + (440 Hz x 0.01059) + (440 Hz x 0.01059)
440.00000 Hz x
000.01059
==========
004.66164 Hz
==========
so we get
440.00000 Hz +
004.66164 Hz +
004.66164 Hz +
004.66164 Hz
===========
453.98492 Hz
===========
If were to Work with C = 256 Hz (a rough approximation so that the numbers will work quickly), the arithmetic this time would be
256 Hz + (256 Hz x 0.01059) + (256 Hz x 0.01059) + (256 Hz x 0.01059)
256.00000 Hz x
000.01059
==========
002.71223 Hz
==========
so we get
256.00000 Hz +
002.71223 Hz +
002.71223 Hz +
002.71223 Hz
===========
263.13669 Hz
===========
If were to work with E = 1319 Hz (a rough approximation so that the numbers will work quickly), the arithmetic this time would be
1319 Hz + (1319 Hz x 0.01059) + (1319 Hz x 0.01059) + (1319 Hz x 0.01059)
1319.00000 Hz x
0000.01059
==========
0013.97432 Hz
==========
so we get
1319.00000 Hz +
0013.97432 Hz +
0013.97432 Hz +
0013.97432 Hz
===========
1360.92296 Hz
===========
This should demonstrate that the ratio of cents to a semitone does not equate to a fixed frequency, but rather to a sliding freq depending on the initial freq.