We have an elderly but much loved PA28 160. We have writtenan automatic weight an balance calculator and keep copies of the performance tables on our web site.
Even with 160hp engine there is no way we would take 3 passsengers. We also have the W & B calculator on a PDA which can be useful.
There are some real odities in the Cherokee which John Davis our local tame instructor pointed out at my biennial, like full fuel and 2 front seat pilots the C of G is in front of the limit so we can (and do) run out of elevator in the flare. Take a look at
www.g-atis.co.uk
Sandown is in a bowl. Runway 23 is significantly uphill and even when lighly loaded the acceleration is slow. The temptation is to try and drag the aeroplane off and having dragged it off the slab wing Cherokee soon gets behind the lift drag curve.
For years I have explained to friends and relatives under extreme peer pressure, why I can only take 2 passengers, thankfully I havnt given in yet, there but for the grace of God go I.