@LauChoKun is correct.
If your trading account balance is £100, then 3% would be £3. If you trade 2000 units (AKA 2 micro lots or 2K) of EUR/USD, then you are risking 20 cents per pip or 0.20 USD.
Therefore, you must convert the 3 GBP you can risk into the USD equivalent using the GBP/USD exchange rate. At the time of this post, the GBP/USD rate is 1.2788 so £3 equates to about $3.84 you can risk.
Divide that by 20 cents per pip on your 2K EUR/USD trade size and you get about 19 pips you can risk on this trade. If you reduce your trade size to 1 micro lot (1K EUR/USD) then you can risk 38 pips and still remain within your 3% risk tolerance.