Installing a TO220 into a TO3 design is simple with a different size fastener for a smaller hole size and 5 in-lb max torque until you get into thermal design ( essential for any EE to learn)
This biggest issue is max Pd depends on the heatsink on the board. If none, then you may have to add one. Consider that the TO220 has a thermal resistance from junction to case of 60'C/W and with 3W it will fry.
So you must define;
- Pd Max ( Vdrop*Imax)
- Tj max ( usually 85'c ) for high MTBF
- T amb max ( maximum internal ambient )
- Air flow or convection restrictions ( helps or hurts Rth )
- space available around one mounting hole that is grounded for heatsink footprint and height
- fastener size ( may need smaller size )
Then compute the max Rjc + Rca for a TO220 + heatsink to meet your above requirements.
Then consider one of these options in order to complete this simple exercise to prepare an ECN with assembly and BOM changes.
Use thermal resistance @ natural and add 5'C/W for Rjc of the TO220 (j=junction, c= case) and you do the math.
Both use case ground, so bending the leads to reach the other 2 holes is easy at right angles with 2 needle nose pliers.
Then like Ohm's Law for voltage, compute the junction temp rise and heatsink Rca @ natural needed to not exceed 85'C ( or higher if approved) at max interior ambient temp.
Pd*(5'C/W + Rth_ca(heatsink)) =< (85'C-Tmax) (internal ambient)