I try to do it every time I make a profit equal to the initial risk - IF - the trend is just as strong or stronger. Unfortunately, that means not every time I trade. Also, you have to accept that on a higher percentage of trades you will make no money or even the initial loss.
So if risk on your first trade T1 is -£100 and take profits at +£100, then you have gained £100 obviously.
If instead you add T2 and move T1’s SL to 0 and give T2 a -£100 risk also, then if the trade goes against you by the same number of pips, you have lost £100. If T2 also gets to +£100 and you add T3 with a -£100 risk, and move T1’s SL to +£100, then if the price moves the same number of pips against you you have now lose nothing - T1 made £100, T2 was break-even and T3 lost £100.
But if you had a TP on T1 and a TP on T2, the same price behaviour would have made you £100 + £100: T3 would have still lost but as its also only £100 you would still be up £100. This can be hard to bear as you see many lost opportunities even if you’re not losing money.