Determining values to convert square wave to sine wave at low frequency
Square wave has odd harmonics 3f,5f,7f at levels of ~ -10dB,-14dB, -17dB ..  

A 1st order filter is -6dB/octave and 3f is 2 octaves above f or -12dB more than the -10db harmonic @ 3f. 

Triangular Current will not be much different to the eye than sinusoidal as the eye is logarithmic .  

You might try something like this.



Then you can always try sine PWM control using a FET switch with current limiting R for each LED.

But the hardest thing it seems to learn is how to define a design spec (before attempting to design it) including the LED datasheets with inputs, Processess and Outputs in a point form Hierarchical format.

IBM used to call this, the HIPO format for design specs.
Simple, Elegant, Complete, Measurable and Verifiable.

If you graduate and do design/test work, you must learn to create Design Specs then Design Verification Tests ( DVT plans and reports) based on these simple specs and record it. This is how Japanese Engineers think and how I did it in Winnipeg and Silicon valley. in early 80's as well as an Aerospace Engineer in late 70's.

If it meets spec. It's perfect. But it has to be a good specs with above Design values, budget and design time. (always double estimated design time)