I'd just like to point out whether it bottlenecks or not is unimportant. What is important is the question of will adding a 980 get you the performance you want?
What I mean is if the i3 gets you 60fps in GTA V with the 980, but you'd get 85fps with an i5- that's a bottleneck, sure. But does it matter? No, not if you wanted 60fps. Do you have a monitor with a refresh rate high enough to take advantage of 85fps? If not, 60fps is all you need.
The 980 will generally outperform the 1050 Ti but in CPU limited games at 1080p the difference might not be worth paying for. It's impossible to say without knowing the cost of the two cards you're comparing.
By the way, if you have an i3 6100 you know you can upgrade to an i5 6xxs quad core...