Does anyone know how to fix this table?table is too big, not very good looking and

The following might help get you started. I have removed all occurences of raisebox,textbf and hline. Instead, I have added a few horizontal lines from the booktabs package.

Side note: The superscript numbers in the lower third of the table look like tablenotes. If that's the case, you might be interested in the threepattable package. I also suggest using the usual label ref mechanism instead of the manual reference to table 3.1.

documentclass{article}
usepackage{booktabs}
begin{document}

begin{table}
    caption{Fundamental Options of the Channel Setup Screen}
    label{tab:PPer} 
    centering 
    begin{tabular}{l l l} 
    toprule 
                             & Impact Hammer & Accelerometer  
    midrule
    Physical Channel ID      & Input1        & Input2,...,16 
    On/Off                   & On            & On 
    Reference                & Yes           & No 
    Channel Group ID         & Vibration     & Vibration 
    Point                    & Impact        & Sensor 
    Direction                & -Z            & +X,+Y,+Z 
    Input Mode               & ICP           & ICP 
    Coupling                 & Single Ended  & Single Ended 
    Measured Quantity        & Force         & Acceleration 
    Electrical Unit          & mV            & mV 
    Actual Sensitivity       & Variable$^1$  & Variable$^2$ 
    Transducer Type          & 086C03        & 356A03 
    Transducer Manufacturer  & PCB           & PCB 
    Serial Number            & $20725$       & Variable$^3$ 
    Range                    & Variable$^4$  & Variable$^5$ 
    midrule
    multicolumn{3}{l}{$^1$ will be different corresponding to different tip table3.1}
    end{tabular}
end{table}
    
end{document}

---

With threeparttable for the tablenotes and addlinespace as a guide to the eye:

documentclass{article}
usepackage{booktabs}
usepackage{threeparttable}
begin{document}

begin{table}
centering 
begin{threeparttable}
    caption{Fundamental Options of the Channel Setup Screen}
    label{tab:PPer} 
    begin{tabular}{l l l} 
    toprule 
                             & Impact Hammer      & Accelerometer     
    midrule
    Physical Channel ID      & Input1             & Input2,...,16     
    On/Off                   & On                 & On                
    Reference                & Yes                & No                
    Channel Group ID         & Vibration          & Vibration         
    Point                    & Impact             & Sensor             addlinespace
    Direction                & -Z                 & +X,+Y,+Z          
    Input Mode               & ICP                & ICP               
    Coupling                 & Single Ended       & Single Ended      
    Measured Quantity        & Force              & Acceleration      
    Electrical Unit          & mV                 & mV                 addlinespace
    Actual Sensitivity       & Variabletnote{1}  & Variabletnote{2} 
    Transducer Type          & 086C03             & 356A03            
    Transducer Manufacturer  & PCB                & PCB               
    Serial Number            & 20725              & Variabletnote{3} 
    Range                    & Variabletnote{4}  & Variabletnote{5} 
    midrule
    end{tabular}
    begin{tablenotes}
    item[1]  will be different corresponding to different tip ref{...}
    item[2] will be different corresponding to different direction and type of accelerometer.
    item[3] description text here
    item[4]
    item[6]
    end{tablenotes}
end{threeparttable}
end{table}
    
end{document}