Ideal Reconstruction of Upsampled Signal
There are some mistakes in the question and your description.

When we sample a signal of frequency $f_m$ with a sampling rate $f_s$, the sampled signal contains the frequencies $f=f_m \pm nf_s$, where $n \in \mathbb{Z} $.
Here the frequencies available in the sampled signal are calculated as,

$f_m=14100 Hz$
 and $f_s=400 Hz$.
we know that, 

$ f=f_m \pm nf_s $, where $n = 0, \pm 1, \pm 2, \pm 3, ... $

ie., $ f= ..., -1500, -1100, -700, -300, 100, 500, 900, 1300, 1700, ... $

When we up sample the signal by 3, the frequencies will become $f/3$.

So here the new frequencies are, $ f_u = ..., -633.33, -500, -366.67, -233.33, -100, 33.33, 166.67, 300, 433.33, 566.67, ... $

After upsampling the signal is reconstructed by using an LPF of cutoff frequency 500.
When we apply LPF to the above signal only the frequencies whose absolute value is below 500 remains and other frequencies will loss.

there for after applying LPF the frequency components available in the signal are,  $ -500, -366.67, -233.33, -100, 33.33, 166.67, 300, 433.33 $.

I hope this solves your problem.