How to filter a signal sampled at different frequencies
If I were you I would define my sampling period as the period present in the interval with the highest sampling frequency. Then I would try to interpolate the sections of the signal that were originally sampled at a lower sampling frequency to obtain a uniform signal of the same sampling frequency. 

Then I would filter the whole signal with the same filter. 

This procedure is often used when you have non-uniform sampling intervals but it might work for you. 

As you say that the signal is very noisy maybe interpolating is not good but you can try to and see how it works.

If you can be 100% sure that decimating the intervals with higher sampling frequency is not going to introduce aliasing you can try to decimate all the intervals to the lowest sampling frequency and avoid interpolating noisy samples.

Maybe even an intermediate solution would be the best. Take every interval to the intermediate sampling frequency, decimating the one with higher sampling frequency (if you are sure there's no aliasing) and interpolating the one with lower sampling frequency.

Since your 3 sampling frequencies are integer multiples of each other you can easily decimate or interpolate from any one to get any other