Recursion function to avoid TLE :

int solve(int n, int index, vector &dp){

if(index == n){
    return 1;
}
if(index > n){
    return 0;
}
if(dp[index]!=-1){
    return dp[index];
}
return dp[index] = solve(n,index+1, dp)+solve(n,index+2, dp)+solve(n,index+3, dp);    

}

int stepPerms(int n) { vectordp(n+1,-1); int ans = solve(n, 0, dp) % mod;

return ans;

}