By the end of 2018 TRX will have the bigger growth, just because it's market cap is the lowest and because new investors tend to buy BTC, then LTC, then alts but the proportion of their buying is favoring alts over big coins, even if they buy more main coin.

For example : invest 1$k : 500$ BTC / 300$ LTC / 200$TRX, even if there is less money invested in TRX, its market cap will grow more than BTC and LTC. BTC/TRX = 120/2,5 = 48 but 500$/200$ = 2,5, wich means that TRX will grow more in % than BTC if the mean investor buy less than 48$ worth of BTC for each 1$ bought of TRX.

Same for LTC/BTC and TRX/LTC.

Of course LTC and TRX are close, if the mean investor buy more than 2,5 LTC for every TRX, LTC will win, but still, the one than needs less money to grow more, so the one with the lowest Market Cap will win.

It's always better to diversifie, just in case, but buying in a proportion that favors the minor market cap is a good idea imho (does not mean to invest more money in the low marcket capped coin, reread if there is a misunderstanding ;-) ). TRX here can also be replaced by a collection of alts, for diversification also.