...because its performance has been proved by dead Luftwaffe pilots over Belgrade...
7 operational IK-3's have claimed 11 unconfirmed victories for the loss of 4 of their own (+ another 3 destroyed on the ground to prevent capture). Percentage-wise that's a great score.
But most of the damage against Luftwaffe over Belgrade has been done by Yugoslav Emil pilots who were able to sneak near German bomber formations unrecognized and wreak havo, while the IK's with their distinctive silhouette were immediately fired upon.
Another setback was the lack of ammo - there was no incendiary ammo available for the machineguns (only ordinary ball ammo) and there was a shortage of 20mm shells so much that after the first day of the war Yugoslav planes (IK's and Emils alike) took off with only 6 20mm rounds per gun instead of the usual full load. This too had an impact on their efficiency.
Breakdown of the 11 IK-3 victories by type:
Ju 87B: 2
Do 17Z: 2
Ju 88A: 3
Me 109E: 2
Me 110D: 2
About half of these were shot down on the first day of the war, 3 more on the second and the rest for the remaining few days of the war.
But despite all of this most important factor we need to consider was the pilot performance. Because in a hands of a good pilot even an essentially inferior machine can defeat the most modern opponent.
It is an enduring myth among the peoples of former Yugoslavia that IK-3 was "the best fighter plane on the whole world". It was most certainly not.
But it was good for what it was - a light fighter of a size similar to MS.406 utilizing a similar engine and armament, and it was clearly superior to 406 in every imaginable performance aspect by some margin.
And especially with the future upgrades - 2nd series and beyond, look at the postwar Ikarus S-49A which was nothing but an IK-3 with a cut-down rear fuselage, bubble canopy, VK-105 engine (itself a development of the Hispano-Suiza) and armed with 1 x 20mm and 2 x .50cal machineguns for an idea about the potential of the initial design - had the war fortunes went some other way the IK-3 would still be a capable fighter well into the second half of the WW2.
Not the "best in the whole world" - but still pretty good.