If at least 1 of the tosses is tails, then 1, 2, or all 3 tosses could be tails. The only possibility this leaves out is the possibility of no tails at all, or all three tosses being heads. So we can simply calculate that probability and subtract it from 1.

The probability of the first toss being heads is 0.4. The probability of the second toss being heads is also 0.4. Same for the third as well. Together, the probability of heads on the first AND second AND third tosses is \(0.4*0.4*0.4 = 0.064\) (generally speaking, in probability "and" means multiply).

So, the probability of this one specific case, all heads, not happening is \(1 - 0.064 = 0.936\).